[BUGFIX] Allow empty end date for scheduler task

An empty end date of a scheduler task is valid and must be allowed.

Resolves: #99175
Resolves: #99878
Releases: main
Change-Id: I7dd1fd515a400e83c7eb71894c973817d0bb6261
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76805


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Josef Glatz <typo3@josefglatz.at>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Josef Glatz <typo3@josefglatz.at>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>