[BUGFIX] Fix undefined array key warning in GroupResolver

Resolves: #95946
Releases: master
Change-Id: Ide10cf2722af3f7cc7b8e4e8830159f7de3510be
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72142


Tested-by: Simon Gilli <typo3@gilbertsoft.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Simon Gilli <typo3@gilbertsoft.org>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>

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

@@ -80,7 +80,7 @@ class GroupResolver
         $validGroups = [];
         foreach ($groupIds as $groupId) {
             // Database did not find the record
-            if (!is_array($foundGroups[$groupId])) {
+            if (!is_array($foundGroups[$groupId] ?? null)) {
                 continue;
             }
             // Record was already processed, continue to avoid adding this group again