[BUGFIX] Improve DataHandler handling for dbType fields

Currently we have some issues with handling database fields with type
"date" or "datetime". Those fields can't have an empty default value
in the database but might have defined default values or null. This
patch adds proper handling for both default value cases.

Resolves: #72654
Resolves: #62660
Releases: master, 7.6, 6.2
Change-Id: I429bf4fdfa32531c0034e8346ee11d3d1fb81242
Reviewed-on: https://review.typo3.org/46426


Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Reviewed-by: Andreas Wolf <andreas.wolf@typo3.org>
Tested-by: Andreas Wolf <andreas.wolf@typo3.org>