baseldap: Return empty string when no effective rights are found
    
    DS returns the string "none" when no rights were found. All clients
    would need to special-case this value when checking the rights.
    Return empty string instead.
    
    https://fedorahosted.org/freeipa/ticket/4359
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>