eliminate NPE on closing restHighLevelClient (if test suite skips all test cases)

Author: vladimirdolzhenko

client/rest-high-level/src/test/java/org/elasticsearch/client/ESRestHighLevelClientTestCase.java

@@ -28,6 +28,7 @@
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.xcontent.XContentBuilder;
 import org.elasticsearch.common.xcontent.XContentType;
+import org.elasticsearch.core.internal.io.IOUtils;
 import org.elasticsearch.ingest.Pipeline;
 import org.elasticsearch.test.rest.ESRestTestCase;
 import org.junit.AfterClass;
@@ -50,7 +51,7 @@ public void initHighLevelClient() throws IOException {
 
     @AfterClass
     public static void cleanupClient() throws IOException {
-        restHighLevelClient.close();
+        IOUtils.close(restHighLevelClient);
         restHighLevelClient = null;
     }