[FOLLOWUP][BUGFIX] EXT:form - handle boolean finisher options

Fix preview rendering.

Change-Id: I0c11304175a0734b2c3690c1e460490e3733bba5
Resolves: #82569
Releases: master, 8.7
Reviewed-on: https://review.typo3.org/55026


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Tested-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>