diff --git a/lib/Controller/AttachmentController.php b/lib/Controller/AttachmentController.php
index 19ec885da8d..e752b5f1253 100644
--- a/lib/Controller/AttachmentController.php
+++ b/lib/Controller/AttachmentController.php
@@ -86,7 +86,7 @@ public function __construct(
 	#[NoAdminRequired]
 	#[PublicPage]
 	#[RequireDocumentSessionOrUserOrShareToken]
-	public function getAttachmentList(?string $shareToken = null): DataResponse {
+	public function getAttachmentList(string $shareToken = ''): DataResponse {
 		$documentId = $this->getDocument()->getId();
 		try {
 			$session = $this->getSession();
@@ -98,7 +98,7 @@ public function getAttachmentList(?string $shareToken = null): DataResponse {
 			$attachments = $this->attachmentService->getAttachmentList($documentId, null, $session, $shareToken);
 		} else {
 			$userId = $this->getUserId();
-			$attachments = $this->attachmentService->getAttachmentList($documentId, $userId, $session, null);
+			$attachments = $this->attachmentService->getAttachmentList($documentId, $userId, $session);
 		}
 
 		return new DataResponse($attachments);
@@ -126,7 +126,7 @@ public function insertAttachmentFile(string $filePath): DataResponse {
 	#[NoAdminRequired]
 	#[PublicPage]
 	#[RequireDocumentSession]
-	public function uploadAttachment(?string $shareToken = null): DataResponse {
+	public function uploadAttachment(string $shareToken = ''): DataResponse {
 		$documentId = $this->getSession()->getDocumentId();
 
 		try {
@@ -193,7 +193,7 @@ private function getUploadedFile(string $key): array {
 	#[PublicPage]
 	#[NoCSRFRequired]
 	#[RequireDocumentSessionOrUserOrShareToken]
-	public function getImageFile(string $imageFileName, ?string $shareToken = null,
+	public function getImageFile(string $imageFileName, string $shareToken = '',
 		int $preferRawImage = 0): DataResponse|DataDownloadResponse {
 		$documentId = $this->getDocument()->getId();
 
@@ -228,7 +228,7 @@ public function getImageFile(string $imageFileName, ?string $shareToken = null,
 	#[PublicPage]
 	#[NoCSRFRequired]
 	#[RequireDocumentSessionOrUserOrShareToken]
-	public function getMediaFile(string $mediaFileName, ?string $shareToken = null): DataResponse|DataDownloadResponse {
+	public function getMediaFile(string $mediaFileName, string $shareToken = ''): DataResponse|DataDownloadResponse {
 		$documentId = $this->getDocument()->getId();
 
 		try {
@@ -259,7 +259,7 @@ public function getMediaFile(string $mediaFileName, ?string $shareToken = null):
 	#[PublicPage]
 	#[NoCSRFRequired]
 	#[RequireDocumentSessionOrUserOrShareToken]
-	public function getMediaFilePreview(string $mediaFileName, ?string $shareToken = null) {
+	public function getMediaFilePreview(string $mediaFileName, string $shareToken = '') {
 		$documentId = $this->getDocument()->getId();
 
 		try {
diff --git a/lib/Controller/SessionController.php b/lib/Controller/SessionController.php
index 9de585d394b..72f5ab1da79 100644
--- a/lib/Controller/SessionController.php
+++ b/lib/Controller/SessionController.php
@@ -58,7 +58,7 @@ public function __construct(
 
 	#[NoAdminRequired]
 	public function create(int $fileId = null, string $file = null): DataResponse {
-		return $this->apiService->create($fileId, $file, null, null);
+		return $this->apiService->create($fileId, $file);
 	}
 
 	#[NoAdminRequired]
diff --git a/lib/Controller/UserApiController.php b/lib/Controller/UserApiController.php
index 435e32f21ac..41f40bb9619 100644
--- a/lib/Controller/UserApiController.php
+++ b/lib/Controller/UserApiController.php
@@ -39,11 +39,12 @@ public function index(string $filter = '', int $limit = 5): DataResponse {
 		// Add joined users to the autocomplete list
 		foreach ($sessions as $session) {
 			$sessionUserId = $session['userId'];
-			if ($sessionUserId !== null && !isset($users[$sessionUserId])) {
-				$displayName = $this->userManager->getDisplayName($sessionUserId);
-				if ($displayName && stripos($displayName, $filter) !== false || stripos($sessionUserId, $filter) !== false) {
-					$users[$sessionUserId] = $displayName;
-				}
+			if ($sessionUserId === null || isset($users[$sessionUserId])) {
+				continue;
+			}
+			$displayName = $this->userManager->getDisplayName($sessionUserId) ?: '';
+			if (stripos($displayName, $filter) !== false || stripos($sessionUserId, $filter) !== false) {
+				$users[$sessionUserId] = $displayName;
 			}
 		}
 
diff --git a/lib/Service/ApiService.php b/lib/Service/ApiService.php
index 288171640e0..a28cf77f08a 100644
--- a/lib/Service/ApiService.php
+++ b/lib/Service/ApiService.php
@@ -70,7 +70,7 @@ public function __construct(IRequest $request,
 		$this->l10n = $l10n;
 	}
 
-	public function create(?int $fileId = null, ?string $filePath = null, ?string $token = null, ?string $guestName = null): DataResponse {
+	public function create(?int $fileId = null, ?string $filePath = null, string $token = '', ?string $guestName = null): DataResponse {
 		try {
 			if ($token) {
 				$file = $this->documentService->getFileByShareToken($token, $this->request->getParam('filePath'));
@@ -86,7 +86,7 @@ public function create(?int $fileId = null, ?string $filePath = null, ?string $t
 				} catch (NotPermittedException $e) {
 					return new DataResponse($this->l10n->t('This file cannot be displayed as download is disabled by the share'), 404);
 				}
-			} elseif ($fileId) {
+			} elseif ($fileId !== null) {
 				try {
 					$file = $this->documentService->getFileById($fileId);
 				} catch (NotFoundException|NotPermittedException $e) {
diff --git a/lib/Service/AttachmentService.php b/lib/Service/AttachmentService.php
index 968bbf03e81..35f8cbc04a7 100644
--- a/lib/Service/AttachmentService.php
+++ b/lib/Service/AttachmentService.php
@@ -214,7 +214,7 @@ private function getMediaFilePreviewFile(string $mediaFileName, File $textFile):
 	 * @throws NotFoundException
 	 * @throws NotPermittedException
 	 */
-	public function getAttachmentList(int $documentId, ?string $userId = null, ?Session $session = null, ?string $shareToken = null): array {
+	public function getAttachmentList(int $documentId, string $userId = '', ?Session $session = null, string $shareToken = ''): array {
 		if ($shareToken) {
 			$textFile = $this->getTextFilePublic($documentId, $shareToken);
 		} elseif ($userId) {
diff --git a/lib/Service/DocumentService.php b/lib/Service/DocumentService.php
index fa3c34e5b8d..f411fc47a8d 100644
--- a/lib/Service/DocumentService.php
+++ b/lib/Service/DocumentService.php
@@ -130,7 +130,7 @@ public function createDocument(File $file): Document {
 			// Do not hard reset if changed from outside since this will throw away possible steps
 			// This way the user can still resolve conflicts in the editor view
 			$stepsVersion = $this->stepMapper->getLatestVersion($document->getId());
-			if ($stepsVersion && ($document->getLastSavedVersion() !== $stepsVersion)) {
+			if ($stepsVersion !== null && ($document->getLastSavedVersion() !== $stepsVersion)) {
 				$this->logger->debug('Unsaved steps, continue collaborative editing');
 				return $document;
 			}
@@ -335,7 +335,7 @@ public function autosave(Document $document, ?File $file, int $version, ?string
 		// Do not save if newer version already saved
 		// Note that $version is the version of the steps the client has fetched.
 		// It may have added steps on top of that - so if the versions match we still save.
-		$stepsVersion = $this->stepMapper->getLatestVersion($documentId)?: 0;
+		$stepsVersion = $this->stepMapper->getLatestVersion($documentId) ?? 0;
 		$savedVersion = $document->getLastSavedVersion();
 		$outdated = $savedVersion > 0 && $savedVersion > $version;
 		if (!$force && ($outdated || $version > (string)$stepsVersion)) {
diff --git a/lib/Service/SessionService.php b/lib/Service/SessionService.php
index e24204e201d..b2ad9390426 100644
--- a/lib/Service/SessionService.php
+++ b/lib/Service/SessionService.php
@@ -236,8 +236,8 @@ public function updateSessionAwareness(Session $session, string $message): Sessi
 		return $this->sessionMapper->update($session);
 	}
 
-	private function getColorForGuestName(string $guestName = null): string {
-		$guestName = $this->userId ?? $guestName;
+	private function getColorForGuestName(string $guestName = ''): string {
+		$guestName = $this->userId ? $guestName : '';
 		$uniqueGuestId = !empty($guestName) ? $guestName : $this->secureRandom->generate(12);
 		$color = $this->avatarManager->getGuestAvatar($uniqueGuestId)->avatarBackgroundColor($uniqueGuestId);
 		return $color->name();