[BUGFIX] Check if storage exists before accessing it

When editing a file mount in the backend, we need to check if
the storage backend actually exists.

Change-Id: I76e677f9b44eea7694005a4939eb9489dc4f71de
Fixes: #57986
Releases: 6.2
Reviewed-on: https://review.typo3.org/29634
Reviewed-by: Wouter Wolters
Reviewed-by: Frans Saris
Reviewed-by: Sebastian Fischer
Tested-by: Sebastian Fischer
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers