[BUGFIX] Fix faulty check for undefined in DateTimePicker

Releases: master
Resolves: #93729
Change-Id: I9dea4a153138e29ae77c9f3e5ae5e202851a09ab
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/68374


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Martin Kutschker <mkutschker-typo3@yahoo.com>
Reviewed-by: Jonas Eberle <flightvision@googlemail.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>