[BUGFIX] Avoid strtotime(null) in BU::getProcessedValue()

BackendUtility has a couple of strtotime() calls that
may end up as strtotime(null). This emits a PHP
E_DEPRECATED error since PHP 8.1. Add a cast.

Resolves: #101811
Resolves: #101805
Releases: main, 12.4, 11.5
Change-Id: I5cca1b2fd3bc0d6792f6d61a59bdbc5b4850d15d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80771


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>