Do not update result of *-config-show with empty server attributes
    
    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>