[BUGFIX] Prevent undefined array key warnings in FormEngine

Resolves: #97475
Releases: 11.5
Change-Id: Iaf7dfb8b0ffde2dae73a75fb106b3433bde46c4a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/74432


Tested-by: core-ci <typo3@b13.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Jochen <rothjochen@gmail.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Henrik Elsner <helsner@dfau.de>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>