refactor(backend): simplify logic for EXTRA_CORS_ORIGINS validation

src/backend/app/config.py

@@ -157,33 +157,30 @@ class Settings(BaseSettings):
     FMTM_DOMAIN: str
     FMTM_DEV_PORT: Optional[str] = "7050"
 
-    EXTRA_CORS_ORIGINS: Optional[Union[str, list[str]]] = []
+    EXTRA_CORS_ORIGINS: Optional[str | list[str]] = []
 
     @field_validator("EXTRA_CORS_ORIGINS", mode="before")
     @classmethod
     def assemble_cors_origins(
         cls,
-        val: Union[str, list[str]],
+        val: Optional[str | list[str]],
         info: ValidationInfo,
     ) -> Union[list[str], str]:
         """Build and validate CORS origins list.
 
         By default, the provided frontend URLs are included in the origins list.
         If this variable used, the provided urls are appended to the list.
         """
-        default_origins = []
-
         # Build default origins from env vars
         url_scheme = "http" if info.data.get("DEBUG") else "https"
         local_server_port = (
             f":{info.data.get('FMTM_DEV_PORT')}" if info.data.get("DEBUG") else ""
         )
-        if frontend_domain := info.data.get("FMTM_DOMAIN"):
-            default_origins = [
-                f"{url_scheme}://{frontend_domain}{local_server_port}",
-                # Also add the xlsform-editor url
-                "https://xlsforms.fmtm.dev",
-            ]
+        default_origins = [
+            f"{url_scheme}://{info.data.get('FMTM_DOMAIN')}{local_server_port}",
+            # Also add the xlsform-editor url
+            "https://xlsforms.fmtm.dev",
+        ]
 
         if val is None:
             return default_origins