[BUGFIX] Avoid PHP 8.0 undefined key in FormEngine

FlexFormSectionContainer raises an exception when
title is not defined.

Releases: main, 11.5
Resolves: #96936
Change-Id: I0ad1a432f11bb2cc36f718ca25f430cf91d94417
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73570

Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>

typo3/sysext/backend/Classes/Form/Container/FlexFormSectionContainer.php

@@ -115,7 +115,7 @@ class FlexFormSectionContainer extends AbstractContainer
         }
 
         $sectionTitle = '';
-        if (!empty(trim($flexFormDataStructureArray['title']))) {
+        if (!empty(trim($flexFormDataStructureArray['title'] ?? ''))) {
             $sectionTitle = $languageService->sL(trim($flexFormDataStructureArray['title']));
         }