[BUGFIX] Fix type error in TcaItemsProcessorFunctions

Cast the variable to string before handing it to preg_replace to avoid
a type error.

Resolves: #97088
Releases: master, 11.5
Change-Id: I60c3eb850049aec3cba7f3bade3a99849683152e
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73778


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>