Fix empty Domain nameserver loads (#769)

* Fix empty Domain nameserver loads

Domains with no nameservers were being loaded from SQL as an empty set instead
of null as they should be.

Discovered this will trying to test updates, so added a test for updates in
the course of it.

core/src/main/java/google/registry/model/domain/DomainContent.java

@@ -380,7 +380,7 @@ public class DomainContent extends EppResource
   // Hibernate needs this in order to populate nsHosts but no one else should ever use it
   @SuppressWarnings("UnusedMethod")
   private void setNsHosts(Set<VKey<HostResource>> nsHosts) {
-    this.nsHosts = nsHosts;
+    this.nsHosts = forceEmptyToNull(nsHosts);
   }
 
   // Hibernate needs this in order to populate gracePeriods but no one else should ever use it