[BUGFIX] Force typecast to string in ArrayBrowser

Make sure that the key in ArrayBrowser::getSearchKeys() is always cast
to string.

Resolves: #91007
Releases: master
Change-Id: I28aa780a92f9425df486cb0dadee3930b4a54f14
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64124


Tested-by: Christian Eßl <indy.essl@gmail.com>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Jonas Eberle <flightvision@googlemail.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Christian Eßl <indy.essl@gmail.com>
Reviewed-by: Jonas Eberle <flightvision@googlemail.com>
Reviewed-by: Benni Mack <benni@typo3.org>