If a server attribute such as DNSSec Key master is unset, None is passed as the attribute value into the upper API layers and displayed in the output of `dnsconfig-show` et al. We should not show this and leave the attribute empty instead. https://fedorahosted.org/freeipa/ticket/5960 Reviewed-By: Pavel Vomacka <pvomacka@redhat.com> Reviewed-By: Martin Basti <mbasti@redhat.com>