[BUGFIX] Fix PHP Warning in caching framework garbage collection

An error was previously shown and is now removed
if no backend type is selected.

Releases: main, 12.4, 11.5
Resolves: #103766
Change-Id: Ia83552de40abffb880244e8b0eb0f4c28f0e6637
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84112


Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>

typo3/sysext/scheduler/Classes/Task/CachingFrameworkGarbageCollectionAdditionalFieldProvider.php

@@ -81,7 +81,7 @@ class CachingFrameworkGarbageCollectionAdditionalFieldProvider extends AbstractA
     {
         $validData = true;
         $availableBackends = $this->getRegisteredBackends();
-        if (is_array($submittedData['scheduler_cachingFrameworkGarbageCollection_selectedBackends'])) {
+        if (isset($submittedData['scheduler_cachingFrameworkGarbageCollection_selectedBackends']) && is_array($submittedData['scheduler_cachingFrameworkGarbageCollection_selectedBackends'])) {
             $invalidBackends = array_diff($submittedData['scheduler_cachingFrameworkGarbageCollection_selectedBackends'], $availableBackends);
             if (!empty($invalidBackends)) {
                 $this->addMessage($this->getLanguageService()->sL('LLL:EXT:scheduler/Resources/Private/Language/locallang.xlf:msg.selectionOfNonExistingCacheBackends'), ContextualFeedbackSeverity::ERROR);