[BUGFIX] Fix notice in ShowFormNoteEditForm

With PHP8 the following notice is thrown:

```
PHP Warning: Trying to access array offset on value of type int in /var/www/html/app/site/public/typo3conf/ext/powermail/Classes/Tca/ShowFormNoteEditForm.php line 224
```
the language uid is in `$this->data['databaseRow']['sys_language_uid']` but as sometimes it is an array and sometimes it is not I suggest the proposed solution to check all

Author: georgringer

Classes/Tca/ShowFormNoteEditForm.php

@@ -219,7 +219,8 @@ protected function getRelatedFormUid(): int
     {
         $flexFormArray = (array)$this->data['databaseRow']['pi_flexform']['data']['main']['lDEF'];
         $formUid = (int)$flexFormArray['settings.flexform.main.form']['vDEF'][0];
-        $formUid = $this->getLocalizedFormUid($formUid, (int)$this->data['databaseRow']['sys_language_uid'][0]);
+        $language = (int)($this->data['databaseRow']['sys_language_uid'][0] ?? $this->data['databaseRow']['sys_language_uid'] ?? 0);
+        $formUid = $this->getLocalizedFormUid($formUid, $language);
         return $formUid;
     }