[ML] Fix NPE by calling super constructor that inits members. (elastic/x-pack-elasticsearch#1763)

Original commit: elastic/x-pack-elasticsearch@ee47ad8322

plugin/src/main/java/org/elasticsearch/xpack/ml/action/IsolateDatafeedAction.java

@@ -171,7 +171,8 @@ public class IsolateDatafeedAction
             readFrom(in);
         }
 
-        public Response() {
+        Response() {
+            super(null, null);
         }
 
         @Override