TYPO3 Core

With #102479 support for persisting datetime_immutable
object has been ensured and covered with a functional
test. However, using a empty string as default for a
not nullable datetime field is invalid for MariaDB and
MySQL which seamed to work until now in test executions.

Something has changed, but had no luck to find any traces
in the mariadb, mysql or corresponding docker hub changelogs
and commits explaining this.

The TYPO3 default TCA schema creates nullable fields with a
null default for `date, datetime and timestamp` anyway, and
therefore we now use that for the test case table structure.

It make sense to test a valid structure if working state is
expected.