eZDFS: don't throw exception with empty string as path

JIRA: https://jira.ez.no/browse/EZP-23409

If a path does not exist in in eZDFSFileHandlerDFSBackend, do not pass an empty string (realpath) to thrown exception.
ok, I feel it would have been easier to read your intention with something like ```$origMountPointPath```, but ok for now.