[BUGFIX] Prevent undefined array key warning when fetching session data

Resolves: #97686
Releases: master, 11.5
Change-Id: I0228126e4b7dd091b6eddc70bbbd8aade6142dcc
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74753


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>

typo3/sysext/frontend/Classes/Authentication/FrontendUserAuthentication.php

@@ -460,7 +460,7 @@ class FrontendUserAuthentication extends AbstractUserAuthentication
         $value = null;
         switch ($type) {
             case 'user':
-                $value = $this->uc[$key];
+                $value = $this->uc[$key] ?? null;
                 break;
             case 'ses':
                 $value = $this->getSessionData($key);