[TEST] close REST test execution context only if not null

The context can be null when REST tests are disabled via sysprop.

src/test/java/org/elasticsearch/test/rest/ElasticsearchRestTests.java

@@ -170,9 +170,11 @@ public class ElasticsearchRestTests extends ElasticsearchIntegrationTest {
 
     @AfterClass
     public static void close() {
+        if (restTestExecutionContext != null) {
             restTestExecutionContext.close();
             restTestExecutionContext = null;
         }
+    }
 
     /**
      * Used to obtain settings for the REST client that is used to send REST requests.