[BUGFIX] Prevent undefined array key exception in FlexFormValueFormatter

The `getProcessedElements` function in `FlexFormValueFormatter`
throws an undefined array key exception, when the given
dataStructure contains a field with a relation table having a
`label_userFunc` and when the given valueStructure does not contain
a value for the field.

This change adds a fallback to an empty string, when
`$valueStructure[$elementKey]['vDEF']` is not set.

Resolves: #104760
Releases: main, 12.4
Change-Id: I133028b341d6df10a32f56858f8be8cfe951625a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/85808


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Stefan Bürk <stefan@buerk.tech>

typo3/sysext/backend/Classes/View/ValueFormatter/FlexFormValueFormatter.php

@@ -196,8 +196,8 @@ class FlexFormValueFormatter
                 if ($relationTable !== null && $labelUserFunction !== null) {
                     $parameters = [
                         'table' => $relationTable,
-                        'row' => BackendUtility::getRecord($relationTable, $valueStructure[$elementKey]['vDEF']),
-                        'title' => $valueStructure[$elementKey]['vDEF'],
+                        'row' => BackendUtility::getRecord($relationTable, $valueStructure[$elementKey]['vDEF'] ?? ''),
+                        'title' => $valueStructure[$elementKey]['vDEF'] ?? '',
                     ];
                     GeneralUtility::callUserFunction($labelUserFunction, $parameters);
                     $processedValue = $parameters['title'];