[BUGFIX] Enforce int for timestamp on DateTime mapping

This patch ensures using an integer value for turning a timestamp
to a DateTime object as otherwise a PHP error is thrown.

Resolves: #84962
Releases: master, 8.7, 7.6
Change-Id: I18e82c8f91f42c29c4014985a1ad11671b5a31f4
Reviewed-on: https://review.typo3.org/56909


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Helmut Hummel <typo3@helhum.io>
Tested-by: Helmut Hummel <typo3@helhum.io>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>