[BUGFIX] Fix PHP 8 warnings in FinisherOptionGenerator

This fixes an array to string conversion and an undefined
array key warning in EXT:form FinisherOptionGenerator.

Resolves: #94686
Releases: master
Change-Id: Ic6265c36ac0f53d70d85baabf9d1881263027207
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70179


Tested-by: core-ci <typo3@b13.com>
Tested-by: Jochen <rothjochen@gmail.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>