Correct RefUpdateUtil to work for an empty BatchRefUpdate

RefUpdateUtil#checkResults should not throw a LockFailureException if
the BatchRefUpdate doesn't contain any commands. Instead, it should
silently do nothing as nothing was executed.

Change-Id: Ie8257f8f9eb487f1242cacf004e05357e3fb6317

java/com/google/gerrit/server/update/RefUpdateUtil.java

@@ -57,6 +57,10 @@ public class RefUpdateUtil {
    */
   @VisibleForTesting
   static void checkResults(BatchRefUpdate bru) throws IOException {
+    if (bru.getCommands().isEmpty()) {
+      return;
+    }
+
     int lockFailure = 0;
     int aborted = 0;
     int failure = 0;

javatests/com/google/gerrit/server/update/RefUpdateUtilTest.java

@@ -53,6 +53,7 @@ public class RefUpdateUtilTest {
 
   @Test
   public void checkBatchRefUpdateResults() throws Exception {
+    checkResults();
     checkResults(OK);
     checkResults(OK, OK);