Allow master to be blank in format validation
Allow master to be blank in format validation

jfqd/powerdns-on-rails,globocom/GloboDNS,Hermanverschooten/powerdns-on-rails,kennethkalmer/powerdns-on-rails,kennethkalmer/powerdns-on-rails,jfqd/powerdns-on-rails,kennethkalmer/powerdns-on-rails,jfqd/powerdns-on-rails,Hermanverschooten/powerdns-on-rails,kennethkalmer/powerdns-on-rails,jfqd/powerdns-on-rails,Hermanverschooten/powerdns-on-rails,globocom/GloboDNS,globocom/GloboDNS,kennethkalmer/powerdns-on-rails,Hermanverschooten/powerdns-on-rails,Hermanverschooten/powerdns-on-rails,globocom/GloboDNS