[DOCS] Prevent PHP 8.1 issues for formValues usage

Resolves: #102669
Resolves: #101768
Releases: main, 12.4
Change-Id: Ie377f122cfc9acba74c1684f68bb4e3434bf12d0
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82186


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>