[BUGFIX] Enforce string type for custom permission key

Make sure that item key for custom permission options are cast
to string before being parsed for invalid characters.

Resolves: #97852
Releases: main, 11.5
Change-Id: Id8ff1997b875171316784d00bdc7bf72702d7645
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75024


Tested-by: core-ci <typo3@b13.com>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Benni Mack <benni@typo3.org>