AD: netlogon_get_domain_info() allow missing arguments and empty results
    
    netlogon_get_domain_info() should not fail if not all parameters can be
    retrieved. It should be the responsibility of the caller to see if the
    needed data is available and act accordingly.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3104
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>