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/main/java/com/google/gerrit/server/notedb/ChangeUpdate.java

@@ -473,12 +473,13 @@ public class ChangeUpdate extends AbstractChangeUpdate {
 
   private boolean isEmpty() {
     return approvals.isEmpty()
-        && reviewers.isEmpty()
+        && changeMessage == null
         && commentsForBase.isEmpty()
         && commentsForPs.isEmpty()
+        && reviewers.isEmpty()
         && status == null
-        && submitRecords == null
+        && subject == null
-        && changeMessage == null;
+        && submitRecords == null;
   }
 
   private static StringBuilder addFooter(StringBuilder sb, FooterKey footer) {

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();