Fix: Typo broke background sync

does not work because of a typo (wrong null coalescing operator). So on
our existing installation, we get an undefined array key error here.

Author: Matthew Toseland

lib/BackgroundJob/SyncSubmissionsWithLinkedFileJob.php

@@ -42,7 +42,7 @@ public function __construct(
 	public function run($argument): void {
 		$oldUser = $this->userSession->getUser();
 		$formId = $argument['form_id'];
-		$attempt = $argument['attempt'] ?: 1;
+		$attempt = $argument['attempt'] ?? 1;
 
 		try {
 			$form = $this->formMapper->findById($formId);