[BUGFIX] Only pass strings to string functions in EXT:backend

Fix some failures that occur if the internal classes in EXT:backend
use strict typing.

Resolves: #101325
Releases: main, 12.4, 11.5
Change-Id: I964b802762991b6409e599d7a746bdbe22740dfb
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80015


Tested-by: Stefan B�rk <stefan@buerk.tech>
Reviewed-by: Stefan B�rk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>