[BUGFIX] Fix warning in BackendUserAuthentication

Check if filemounts are available before accessing those.

Resolves: #95780
Releases: master
Change-Id: I0f475a7add63eec1ec105760f745dd09113867c4
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71993
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Jochen <rothjochen@gmail.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>

typo3/sysext/core/Classes/Authentication/BackendUserAuthentication.php

@@ -1394,7 +1394,7 @@ public function getFileMountRecords()
         $connectionPool = GeneralUtility::makeInstance(ConnectionPool::class);
 
         // Processing file mounts (both from the user and the groups)
-        $fileMounts = array_unique(GeneralUtility::intExplode(',', $this->groupData['filemounts'], true));
+        $fileMounts = array_unique(GeneralUtility::intExplode(',', $this->groupData['filemounts'] ?? '', true));
 
         // Limit file mounts if set in workspace record
         if ($this->workspace > 0 && !empty($this->workspaceRec['file_mountpoints'])) {