Disallow empty nameservers for domains in TLDs with whitelist

If a TLD has a whitelist on nameservers, domains in such TLD must have
at least one nameserver. Therefore creating domains with empty nameserver
is forbidden, as well as deleting the last nameserver on a domain. We
enforce this policy by checking the number of nameservers for the new resource
to makesure it is not zero if a whitelist exists.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=127318320

java/google/registry/flows/domain/DomainUpdateFlow.java

@@ -59,6 +59,7 @@ import org.joda.time.DateTime;
  * @error {@link DomainFlowUtils.MissingContactTypeException}
  * @error {@link DomainFlowUtils.MissingTechnicalContactException}
  * @error {@link DomainFlowUtils.NameserversNotAllowedException}
+ * @error {@link DomainFlowUtils.NameserversNotSpecifiedException}
  * @error {@link DomainFlowUtils.RegistrantNotAllowedException}
  * @error {@link DomainFlowUtils.TooManyDsRecordsException}
  * @error {@link DomainFlowUtils.TooManyNameserversException}