[BUGFIX] Validators are skipped for empty values

If no data is submitted in a form field, but e.g. the
RequiredValidator is configured, the validation is not
executed.
This behavior has been changed during TYPO3 CMS 6.2
development and needs to be changed individually for
custom validators (see issue #48767).

Resolves: #60144
Change-Id: I14b438cb6a84cf75d4e8d7609ae7cc40ce9bdcaa
Reviewed-on: https://review.typo3.org/31444
Reviewed-by: Sebastian Fischer
Tested-by: Sebastian Fischer