[BUGFIX] Rework Flexform data handling in TcaFlexPrepare

The previous code in `TcaFlexPrepare->initializeDataValues()` was pretty
optimistic regarding the incoming data and may booted XML parsing on
empty strings and did not consider `null` values.

The code is changed to check now whether the incoming data is not null
and not an empty string before using it at all.

Resolves: #104962
Releases: main, 13.4
Change-Id: If8dbffdc82b1a1e53409e0417d5ddd7a25ed6946
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/87684
Reviewed-by: Andreas Kienast <akienast@scripting-base.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Andreas Kienast <akienast@scripting-base.de>
Tested-by: core-ci <typo3@b13.com>

Author: georgringer

typo3/sysext/backend/Classes/Form/FormDataProvider/TcaFlexPrepare.php

@@ -93,18 +93,17 @@ protected function initializeDataStructure(array $result, string $fieldName): ar
      */
     protected function initializeDataValues(array $result, string $fieldName): array
     {
-        if (!array_key_exists($fieldName, $result['databaseRow'])) {
-            $result['databaseRow'][$fieldName] = '';
-        }
         $valueArray = [];
-        if (isset($result['databaseRow'][$fieldName])) {
-            $valueArray = $result['databaseRow'][$fieldName];
-        }
-        if (!is_array($result['databaseRow'][$fieldName])) {
-            $valueArray = GeneralUtility::xml2array($result['databaseRow'][$fieldName]);
-        }
-        if (!is_array($valueArray)) {
-            $valueArray = [];
+
+        if (isset($result['databaseRow'][$fieldName]) && $result['databaseRow'][$fieldName] !== '') {
+            if (is_array($result['databaseRow'][$fieldName])) {
+                $valueArray = $result['databaseRow'][$fieldName];
+            } else {
+                $valueArray = GeneralUtility::xml2array($result['databaseRow'][$fieldName]);
+                if (!is_array($valueArray)) {
+                    $valueArray = [];
+                }
+            }
         }
         if (!isset($valueArray['data'])) {
             $valueArray['data'] = [];