Fixed #21750 -- Fixed regression introduced by 4befb30.

Validating STATIC_ROOT in StaticFilesStorage.__init__ turned out to
be problematic - especially for tests - because the storage refuses
to work even if there are no actual interactions with the file system.

Originally the validation happened only in the path method, but that
didn't work as expected because the call to FileSystemStorage.__init__
wold replace the empty value by a valid path. The new approach is to
move back the check to the StaticFilesStorage.path method, but ensure
that the location attribute remains None after the call to super.

Refs #21581.