[BUGFIX] Fix date conversion of neg timestamps

Date conversion of TCA fields with eval "date" or "datetime" is now also
handled correctly for dates before 1970, thus having a negative
timestamp. Timezone offset is now also applied for those negative
timestamps, in order to prevent erroneous data for dates before 1970.

Furthermore validation handling for dates with zero timestamp
(01.01.1970 midnight UTC) is fixed.

Resolves: #73871
Releases: master, 7.6
Change-Id: Iffa2f12c6941fe17b956202fe9c49f811b1b0539
Reviewed-on: https://review.typo3.org/47025


Reviewed-by: Andreas Wolf <andreas.wolf@typo3.org>
Tested-by: Andreas Wolf <andreas.wolf@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>