[BUGFIX] Allow 1970-01-01 as native datetime input

The TCA processing temporary converts native datetime inputs to
timestamps for some checks.
The start of the unix epoch (1970-01-01 00:00) corresponds to the
0 timestamp. Conditionally checks interpreted that as false,
resulting the value to be the default value.
This fix also prevents empty date values from being set to
MySQL-specific values (0000-00-00 00:00:00).

Resolves: #92900
Releases: main, 12.4
Change-Id: I979d59f4abe3dd73117a9e135a74ae3036192d9b
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/79812


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>