[BUGFIX] EXT:form - Validation of radiogroup/ checkboxgroup elements

The "required" validator has to check array values.

Resolves: #70210
Releases: master
Change-Id: I58b8fb93d7e684854d1834c7994c837922467f82
Reviewed-on: https://review.typo3.org/43710


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>