[BUGFIX] Skip adding child elements if a nested form element is unknown

Skip adding child elements within the form factory if a nested form
element is unknown.

Resolves: #84833
Releases: master, 8.7
Change-Id: I1cbb87b4c460bd45b58aa6bcca27a88c61a7e92f
Reviewed-on: https://review.typo3.org/56773


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Tested-by: Mathias Brodala <mbrodala@pagemachine.de>