Clean up code in findEditorLinkedDescriptors()

As noted on PR #1277, the code looks weird and I can't explain what I
smoke while writing it. Fixed the code to do what actually was meant.
This is mostly paranoia code, as usually input.getName() is not null.

See #1277
See eclipse-platform/eclipse.platform.ui#1747

team/bundles/org.eclipse.compare/compare/org/eclipse/compare/internal/CompareUIPlugin.java

@@ -1118,12 +1118,11 @@ private static void ensureTextIsLast(LinkedHashSet<ViewerDescriptor> result) {
 	 */
 	Set<ViewerDescriptor> findEditorLinkedDescriptors(String fileName, IContentType contentType,
 			boolean firstIsEnough) {
-		if (fileName == null) {
-			if (contentType == null) {
-				contentType = fgContentTypeManager.findContentTypeFor(fileName);
-			} else {
-				return Collections.emptySet();
-			}
+		if (fileName == null && contentType == null) {
+			return Collections.emptySet();
+		}
+		if (contentType == null) {
+			contentType = fgContentTypeManager.findContentTypeFor(fileName);
 		}
 
 		LinkedHashSet<ViewerDescriptor> viewers = fContentMergeViewers.getAll().stream()