Improve Datastore efficiency of duplicate contact messages

I should have caught this in the review, but [] is loading *ALL*
contacts individually from Datastore on every domain update. This will add a
large number of Datastore round trips and thus significantly reduce update
performance.

This CL changes the behavior to *ONLY* load contacts when there is a duplicate
(which is needed to determine the contact's display name to generate the error
message), and loads all of them in a single batch rather than individually.

This also makes some minor changes around domain getters returning empty sets
instead of null.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=233128140

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

@@ -786,7 +786,7 @@ public class DomainUpdateFlowTest extends ResourceFlowTestCase<DomainUpdateFlow,
     assertThat(thrown.getResult().getMsg())
         .isEqualTo(
             "More than one contact for a given role is not allowed: "
-                + "contacts [foo, mak21] have same role [tech]");
+                + "role [tech] has contacts [foo, mak21]");
   }
 
   @Test
@@ -903,10 +903,12 @@ public class DomainUpdateFlowTest extends ResourceFlowTestCase<DomainUpdateFlow,
     persistReferencedEntities();
     persistDomain();
     EppException thrown = assertThrows(DuplicateContactForRoleException.class, this::runFlow);
-    assertThat(thrown.getMessage())
-        .isEqualTo("More than one contact for a given role is not allowed: "
-            + "contacts [mak21, sh8013] have same role [billing], "
-            + "contacts [mak21, sh8013] have same role [tech]");
+    assertThat(thrown)
+        .hasMessageThat()
+        .isEqualTo(
+            "More than one contact for a given role is not allowed: "
+                + "role [billing] has contacts [mak21, sh8013], "
+                + "role [tech] has contacts [mak21, sh8013]");
     assertAboutEppExceptions().that(thrown).marshalsToXml();
   }