[BUGFIX] Avoid undefined array key in TextMenuContentObject->writeMenu()

Resolves: #96377
Releases: main, 11.5
Change-Id: I98dfe39cf04468d54ee979afa34e7dd3bd40acf6
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72793
Tested-by: core-ci <typo3@b13.com>
Tested-by: Jochen <rothjochen@gmail.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>

typo3/sysext/frontend/Classes/ContentObject/Menu/TextMenuContentObject.php

@@ -111,7 +111,7 @@ public function writeMenu()
             }
             // ATagBeforeWrap processing:
             if ($this->I['val']['ATagBeforeWrap'] ?? false) {
-                $wrapPartsBefore = explode('|', $this->I['val']['linkWrap']);
+                $wrapPartsBefore = explode('|', $this->I['val']['linkWrap'] ?? '');
                 $wrapPartsAfter = ['', ''];
             } else {
                 $wrapPartsBefore = ['', ''];