[BUGFIX] Check if header variable is set before using it

Avoid a TypeError exception in NewContentElementController in
newer PHP versions because of an undefined array access.

Resolves: #89860
Releases: master, 9.5
Change-Id: I5728638a3f5800588f17bef202fe5814e79d7f2f
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62540
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Tobias D. <tobias.doll@snk.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Benni Mack <benni@typo3.org>

typo3/sysext/backend/Classes/Controller/ContentElement/NewContentElementController.php

@@ -270,7 +270,7 @@ protected function prepareContent(string $clientContext): void
 
             foreach ($wizardItems as $wizardKey => $wInfo) {
                 $wizardOnClick = '';
-                if ($wInfo['header']) {
+                if (isset($wInfo['header'])) {
                     $menuItems[] = [
                         'label' => $wInfo['header'] ?: '-',
                         'content' => ''