[BUGFIX] Prevent PHP warning in menu data processors

Check if the decoded menu is iterable,
before passing it to "foreach".

Note: No early return is used, since
this would be a change in behaviour.

Resolves: #96563
Releases: main, 11.5
Change-Id: If291ace4f6fc9c897ecacc73a1e6cdd0c98617e9
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73091


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>