[BUGFIX] FormEngine: Fix type=group with internal_type=folder

Restore support for group type fields with internal type of `folder`.

Resolves: #70449
Releases: master
Change-Id: I6cfea6c74c003d64fbbbe7d8904420c757f044d5
Reviewed-on: https://review.typo3.org/44700


Tested-by: Manfred Rutschmann <manfred@dslr-seite.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>