[BUGFIX] Exception using cObject FORM in TypoScript

Through a bug introduced by #31120, an Exception is thrown when a
TypoScript FORM object should be rendered.

Change-Id: Ie67484fd014620ea229a7bee8bfebd063a3b57ae
Fixes: #31572
Releases: 6.2, 6.1, 6.0, 4.7
Reviewed-on: https://review.typo3.org/6537
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind