Ticket 50063 - Crash after attempting to restore a single backend
    
    Description:  While we do not support backup/restore of individual backends,
                  it should not crash the server either.  PR_OpenDir will crash
                  if the file name is NULL, so this fix just prevents the crash
                  by returning an error if the filename is NULL.
    
    https://pagure.io/389-ds-base/issue/50063
    
    Reviewed by: firstyear & tbordaz(Thanks!!)