[BUGFIX] Fix warning on array key updateReferenceIndex

Patch fixes warning on PHP 8.1 in extbase Backend class.

Resolves: #98149
Releases: main, 11.5
Change-Id: I6f32f67672332758cc02708d07481b93edb09b82
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75474


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>

typo3/sysext/extbase/Classes/Persistence/Generic/Backend.php

@@ -808,7 +808,7 @@ class Backend implements BackendInterface, SingletonInterface
         $this->eventDispatcher->dispatch(new EntityUpdatedInPersistenceEvent($object));
 
         $frameworkConfiguration = $this->configurationManager->getConfiguration(ConfigurationManagerInterface::CONFIGURATION_TYPE_FRAMEWORK);
-        if ($frameworkConfiguration['persistence']['updateReferenceIndex'] === '1') {
+        if (($frameworkConfiguration['persistence']['updateReferenceIndex'] ?? '') === '1') {
             $this->referenceIndex->updateRefIndexTable($dataMap->getTableName(), $row['uid']);
         }
         return true;