Fix type of PreviewController::$userId

Can be null if not logged in; currently crashes

Signed-off-by: Varun Patil <varunpatil@ucla.edu>

core/Controller/PreviewController.php

@@ -40,7 +40,7 @@ use OCP\IPreview;
 use OCP\IRequest;
 
 class PreviewController extends Controller {
-	private string $userId;
+	private ?string $userId;
 	private IRootFolder $root;
 	private IPreview $preview;