LIVY-236. Fix NPE issue in Job API when return value is Void

Closes apache#215

client-http/src/main/java/com/cloudera/livy/client/http/JobHandleImpl.java

@@ -235,9 +235,11 @@ public void run() {
 
         switch (status.state) {
           case SUCCEEDED:
-            @SuppressWarnings("unchecked")
-            T localResult = (T) serializer.deserialize(ByteBuffer.wrap(status.result));
-            result = localResult;
+            if (status.result != null) {
+              @SuppressWarnings("unchecked")
+              T localResult = (T) serializer.deserialize(ByteBuffer.wrap(status.result));
+              result = localResult;
+            }
             finished = true;
             break;
 

integration-test/src/test/scala/com/cloudera/livy/test/JobApiIT.scala

@@ -190,6 +190,13 @@ class JobApiIT extends BaseIntegrationTestSuite with BeforeAndAfterAll with Logg
     assert(result === "foo")
   }
 
+  test("return null should not throw NPE") {
+    assume(client2 != null, "Client not active")
+
+    val result = waitFor(client2.submit(new VoidJob()))
+    assert(result === null)
+  }
+
   test("destroy the session") {
     assume(client2 != null, "Client not active.")
     client2.stop(true)

test-lib/src/main/java/com/cloudera/livy/test/jobs/VoidJob.java

@@ -0,0 +1,29 @@
+/*
+ * Licensed to Cloudera, Inc. under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  Cloudera, Inc. licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.cloudera.livy.test.jobs;
+
+import com.cloudera.livy.Job;
+import com.cloudera.livy.JobContext;
+
+public class VoidJob implements Job<Void> {
+  @Override
+  public Void call(JobContext jc) {
+    return null;
+  }
+}