[TASK] Fix PHP 8 compatibility in DataHandler

In case you add a new TCA field of type "slug" in the pages table
but without a config "eval", you get an exception in the backend.

Resolves: #101640
Related: #93640
Releases: main, 12.4, 11.5
Change-Id: Ia9fc65707c581c288101eb9d54957aa9d025b98a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80496


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>