[TASK] Use stricter checks in DataHandler

Resolves: #63993
Releases: master
Change-Id: I157be570d1d226685511f83e37fe287e244b9fb2
Reviewed-on: http://review.typo3.org/35329
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Frank Nägler <typo3@naegler.net>
Tested-by: Frank Nägler <typo3@naegler.net>