Don't fail on removing star labels from change without star labels

Setting no star labels on a change that doesn't have star labels should
be a no-op, but it currently fails with 500 Internal Server Error
because it attempts to delete a ref that doesn't exist.

Change-Id: Id180e4ff4f928aa229e29ed030b7af640e4605ff
Signed-off-by: Edwin Kempin <ekempin@google.com>
(cherry picked from commit 036751e0e3)

gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/api/accounts/AccountIT.java

@@ -504,6 +504,17 @@ public class AccountIT extends AbstractDaemonTest {
                 ImmutableSet.of(DEFAULT_LABEL, "invalid label", "blue", "another invalid label")));
   }
 
+  @Test
+  public void deleteStarLabelsFromChangeWithoutStarLabels() throws Exception {
+    PushOneCommit.Result r = createChange();
+    String triplet = project.get() + "~master~" + r.getChangeId();
+    assertThat(gApi.accounts().self().getStars(triplet)).isEmpty();
+
+    gApi.accounts().self().setStars(triplet, new StarsInput());
+
+    assertThat(gApi.accounts().self().getStars(triplet)).isEmpty();
+  }
+
   @Test
   public void starWithDefaultAndIgnoreLabel() throws Exception {
     PushOneCommit.Result r = createChange();

gerrit-server/src/main/java/com/google/gerrit/server/StarredChangesUtil.java

@@ -477,6 +477,11 @@ public class StarredChangesUtil {
 
   private void deleteRef(Repository repo, String refName, ObjectId oldObjectId)
       throws IOException, OrmException {
+    if (ObjectId.zeroId().equals(oldObjectId)) {
+      // ref doesn't exist
+      return;
+    }
+
     RefUpdate u = repo.updateRef(refName);
     u.setForceUpdate(true);
     u.setExpectedOldObjectId(oldObjectId);