[BUGFIX] Fix warning when FlexForm sheet has no fields

This patch fixes a warning on PHP 8.1, when a FlexForm has
a sheet with no fields in it.

Resolves: #98166
Releases: main, 11.5
Change-Id: Iec814a10bb755fb78a8de95b432d8cb6042a07d3
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75490


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>

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

@@ -57,7 +57,7 @@ class FlexFormTabsContainer extends AbstractContainer
         $tabCounter = 0;
         $tabElements = [];
         foreach ($flexFormDataStructureArray['sheets'] as $sheetName => $sheetDataStructure) {
-            $flexFormRowSheetDataSubPart = $flexFormRowData['data'][$sheetName]['lDEF'] ?: [];
+            $flexFormRowSheetDataSubPart = $flexFormRowData['data'][$sheetName]['lDEF'] ?? [];
 
             if (!is_array($sheetDataStructure['ROOT']['el'])) {
                 $resultArray['html'] .= LF . 'No Data Structure ERROR: No [\'ROOT\'][\'el\'] found for sheet "' . $sheetName . '".';