adtrustinstance: make sure NetBIOS name defaults are set properly
    
    Some tools may pass None as NetBIOS name if not put explicitly by a
    user. This meant to use default NetBIOS name generator based on the
    domain (realm) name. However, this wasn't done properly, so None is
    passed later to python-ldap and it rejects such LDAP entry.
    
    Fixes: https://pagure.io/freeipa/issue/9514
    
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>