[BUGFIX] Prevent TypeErrors in FormEngine for missing elements

Resolves: #103391
Releases: main, 12.4
Change-Id: I48d55c7b66669f9836886f257aefba2171b6910e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/83455


Tested-by: Jochen Roth <rothjochen@gmail.com>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Jochen Roth <rothjochen@gmail.com>
Tested-by: Andreas Kienast <a.fernandez@scripting-base.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Andreas Kienast <a.fernandez@scripting-base.de>