Improve the error when trying to delete the registrant contact

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=196708902

docs/flows.md

@@ -956,6 +956,7 @@ the domain to convert to a normal create and be billed for accordingly.
         is not free.
     *   Admin contact is required.
     *   Technical contact is required.
+    *   Registrant is required.
 *   2004
     *   The specified status value cannot be set by clients.
     *   The fees passed in the transform command do not match the fees that will

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

@@ -59,6 +59,7 @@ import google.registry.flows.custom.DomainUpdateFlowCustomLogic;
 import google.registry.flows.custom.DomainUpdateFlowCustomLogic.AfterValidationParameters;
 import google.registry.flows.custom.DomainUpdateFlowCustomLogic.BeforeSaveParameters;
 import google.registry.flows.custom.EntityChanges;
+import google.registry.flows.domain.DomainFlowUtils.MissingRegistrantException;
 import google.registry.model.ImmutableObject;
 import google.registry.model.billing.BillingEvent;
 import google.registry.model.billing.BillingEvent.Reason;
@@ -117,6 +118,7 @@ import org.joda.time.DateTime;
  * @error {@link DomainFlowUtils.MissingAdminContactException}
  * @error {@link DomainFlowUtils.MissingContactTypeException}
  * @error {@link DomainFlowUtils.MissingTechnicalContactException}
+ * @error {@link DomainFlowUtils.MissingRegistrantException}
  * @error {@link DomainFlowUtils.NameserversNotAllowedForTldException}
  * @error {@link DomainFlowUtils.NameserversNotSpecifiedForTldWithNameserverWhitelistException}
  * @error {@link DomainFlowUtils.NameserversNotAllowedForDomainException}
@@ -258,6 +260,7 @@ public final class DomainUpdateFlow implements TransactionalFlow {
     checkSameValuesNotAddedAndRemoved(add.getContacts(), remove.getContacts());
     checkSameValuesNotAddedAndRemoved(add.getStatusValues(), remove.getStatusValues());
     Change change = command.getInnerChange();
+    validateRegistrantIsntBeingRemoved(change);
     Optional<SecDnsUpdateExtension> secDnsUpdate =
         eppInput.getSingleExtension(SecDnsUpdateExtension.class);
     DomainResource.Builder domainBuilder =
@@ -330,6 +333,12 @@ public final class DomainUpdateFlow implements TransactionalFlow {
         .build();
   }
 
+  private void validateRegistrantIsntBeingRemoved(Change change) throws EppException {
+    if (change.getRegistrantContactId() != null && change.getRegistrantContactId().isEmpty()) {
+      throw new MissingRegistrantException();
+    }
+  }
+
   private void validateNewState(DomainResource newDomain) throws EppException {
     validateNoDuplicateContacts(newDomain.getContacts());
     validateRequiredContactsPresent(newDomain.getRegistrant(), newDomain.getContacts());

java/google/registry/model/domain/DomainCommand.java

@@ -27,6 +27,7 @@ import static google.registry.util.CollectionUtils.nullToEmptyImmutableCopy;
 import static google.registry.util.CollectionUtils.union;
 
 import com.google.common.base.MoreObjects;
+import com.google.common.base.Strings;
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSet;
 import com.googlecode.objectify.Key;
@@ -387,7 +388,7 @@ public class DomainCommand {
       /** Creates a copy of this {@link Change} with hard links to hosts and contacts. */
       Change cloneAndLinkReferences(DateTime now) throws InvalidReferencesException {
         Change clone = clone(this);
-        clone.registrant = clone.registrantContactId == null
+        clone.registrant = Strings.isNullOrEmpty(clone.registrantContactId)
             ? null
             : getOnlyElement(
                 loadByForeignKeys(

javatests/google/registry/flows/domain/DomainUpdateFlowTest.java

@@ -63,6 +63,7 @@ import google.registry.flows.domain.DomainFlowUtils.LinkedResourcesDoNotExistExc
 import google.registry.flows.domain.DomainFlowUtils.MaxSigLifeChangeNotSupportedException;
 import google.registry.flows.domain.DomainFlowUtils.MissingAdminContactException;
 import google.registry.flows.domain.DomainFlowUtils.MissingContactTypeException;
+import google.registry.flows.domain.DomainFlowUtils.MissingRegistrantException;
 import google.registry.flows.domain.DomainFlowUtils.MissingTechnicalContactException;
 import google.registry.flows.domain.DomainFlowUtils.NameserversNotAllowedForDomainException;
 import google.registry.flows.domain.DomainFlowUtils.NameserversNotAllowedForTldException;
@@ -185,6 +186,16 @@ public class DomainUpdateFlowTest extends ResourceFlowTestCase<DomainUpdateFlow,
     doSuccessfulTest();
   }
 
+  @Test
+  public void testFailure_emptyRegistrant() throws Exception {
+    setEppInput("domain_update_empty_registrant.xml");
+    persistReferencedEntities();
+    persistDomain();
+    MissingRegistrantException thrown =
+        assertThrows(MissingRegistrantException.class, this::runFlow);
+    assertAboutEppExceptions().that(thrown).marshalsToXml();
+  }
+
   private void doSunrushAddTest(
       BillingEvent.OneTime sunrushAddBillingEvent,
       UserPrivileges userPrivileges,

javatests/google/registry/flows/domain/testdata/domain_update_empty_registrant.xml

@@ -0,0 +1,32 @@
+<epp xmlns="urn:ietf:params:xml:ns:epp-1.0">
+  <command>
+    <update>
+      <domain:update
+       xmlns:domain="urn:ietf:params:xml:ns:domain-1.0">
+        <domain:name>example.tld</domain:name>
+        <domain:add>
+          <domain:ns>
+            <domain:hostObj>ns2.example.foo</domain:hostObj>
+          </domain:ns>
+          <domain:contact type="tech">mak21</domain:contact>
+          <domain:status s="clientHold"
+           lang="en">Payment overdue.</domain:status>
+        </domain:add>
+        <domain:rem>
+          <domain:ns>
+            <domain:hostObj>ns1.example.foo</domain:hostObj>
+          </domain:ns>
+          <domain:contact type="tech">sh8013</domain:contact>
+          <domain:status s="clientUpdateProhibited"/>
+        </domain:rem>
+        <domain:chg>
+          <domain:registrant/>
+          <domain:authInfo>
+            <domain:pw>2BARfoo</domain:pw>
+          </domain:authInfo>
+        </domain:chg>
+      </domain:update>
+    </update>
+    <clTRID>ABC-12345</clTRID>
+  </command>
+</epp>