[BUGFIX] EXT:form - Fix PHP warning in debug mode

Avoid PHP warning on empty filters configuration.

Resolves: #76642
Releases: master, 7.6
Change-Id: Ia1171867acd150a1fdd595bcea1bda58c98fac36
Reviewed-on: https://review.typo3.org/48585


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Tested-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>