Support empty ChangeUpdates

To create a change in the notedb and ensure the meta ref exists, we
need a root commit, but thus far we don't have anything else to put in
the root commit, so it is effectively empty. Ensure this code path
works.

Change-Id: I3e5496c7019f083401ef24e430e2b9f89c9936b4

gerrit-server/src/test/java/com/google/gerrit/server/notedb/ChangeNotesTest.java

@@ -22,6 +22,8 @@ import static java.nio.charset.StandardCharsets.UTF_8;
 import static java.util.concurrent.TimeUnit.MILLISECONDS;
 import static java.util.concurrent.TimeUnit.SECONDS;
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
 
 import com.google.common.collect.ImmutableList;
@@ -618,6 +620,20 @@ public class ChangeNotesTest {
         Iterables.getOnlyElement(notes.getSubmitRecords()));
   }
 
+  @Test
+  public void emptyChangeUpdate() throws Exception {
+    ChangeUpdate update = newUpdate(newChange(), changeOwner);
+    update.commit();
+    assertNull(update.getRevision());
+  }
+
+  public void emptyExceptSubject() throws Exception {
+    ChangeUpdate update = newUpdate(newChange(), changeOwner);
+    update.setSubject("Create change");
+    update.commit();
+    assertNotNull(update.getRevision());
+  }
+
   @Test
   public void multipleUpdatesInBatch() throws Exception {
     Change c = newChange();