Added test for null result in BatchResult.

Author: mderka

gcloud-java-core/src/test/java/com/google/cloud/BatchResultTest.java

@@ -45,6 +45,8 @@ public void testSuccess() {
     }
     result.success(true);
     assertTrue(result.get());
+    // test that null is allowed
+    result.success(null);
   }
 
   @Test