[BUGFIX] EXT:form - avoid php warnings

Avoid php warnings if no individual validation errors are defined.

Resolves: #80439
Releases: master
Change-Id: Idc3f566eb922afe3fc167d5738da54c09f78f83e
Reviewed-on: https://review.typo3.org/52152


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Patrick Broens <patrick@patrickbroens.nl>
Tested-by: Patrick Broens <patrick@patrickbroens.nl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>