[BUGFIX] Prevent undefined array key in BackendUserAuthentication

The file mount identifier, introduced in #97922
might be invalid, e.g. in case the upgrade wizard
was not executed yet.

To prevent an undefined array key warning, the
identifier is now properly checked, like already
done in the StoragePermissionsAspect.

Resolves: #99560
Related: #97922
Releases: main
Change-Id: Iba75d2a6792a5e619a5ea51d9664e511c8ad46ea
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77400


Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>