[BUGFIX] Check for array key in FlexFormService

Resolves: #99846
Releases: main, 11.5
Change-Id: Ie635412ce817f537c34143aac493e4038e3b9afc
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77755


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