Add null param check for StandardPodUtils

Add unit test for StandardPodUtils

spring-cloud-kubernetes-core/src/main/java/org/springframework/cloud/kubernetes/StandardPodUtils.java

@@ -36,6 +36,10 @@ public class StandardPodUtils implements PodUtils {
     private Supplier<Pod> current;
 
     public StandardPodUtils(KubernetesClient client) {
+        if (client == null) {
+            throw new IllegalArgumentException("Must provide an instance of KubernetesClient");
+        }
+
         this.client = client;
         this.hostName = System.getenv(HOSTNAME);
         this.current = LazilyInstantiate.using(() -> internalGetPod());

spring-cloud-kubernetes-core/src/test/java/org/springframework/cloud/kubernetes/StandardPodUtilsTest.java

@@ -0,0 +1,29 @@
+/*
+ *     Copyright (C) 2016 to the original authors.
+ *
+ *     Licensed 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 org.springframework.cloud.kubernetes;
+
+import org.junit.Test;
+
+public class StandardPodUtilsTest {
+
+    @Test(expected = IllegalArgumentException.class)
+    public void constructorThrowsIllegalArgumentExceptionWhenKubeClientNull() {
+        // expect an IllegalArgumentException if KubernetesClient argument is
+        // null
+        new StandardPodUtils(null);
+    }
+
+}