Improve closing mock webserver when failed to start (#52943)

Fix NPE when closing a webserver that hasn't started correctly.

This can happen when ssl context isn't initialized. The server instance is then never set,
which causes an NPE that masks the actual failure.

Example stacktrace that would mask an actual failure:

```
java.lang.NullPointerException
	at org.elasticsearch.test.http.MockWebServer.close(MockWebServer.java:271)
	at org.elasticsearch.xpack.watcher.test.integration.HttpSecretsIntegrationTests.cleanup(HttpSecretsIntegrationTests.java:70)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
```

x-pack/plugin/core/src/test/java/org/elasticsearch/test/http/MockWebServer.java

@@ -268,11 +268,13 @@ public class MockWebServer implements Closeable {
         logger.debug("[{}:{}] Counting down all latches before terminating executor", getHostName(), getPort());
         latches.forEach(CountDownLatch::countDown);
 
+        if (server != null) {
             if (server.getExecutor() instanceof ExecutorService) {
                 terminate((ExecutorService) server.getExecutor());
             }
             server.stop(0);
         }
+    }
 
     /**
      * Helper method to return the first 20 chars of a request's body

x-pack/plugin/watcher/src/test/java/org/elasticsearch/xpack/watcher/actions/webhook/WebhookHttpsIntegrationTests.java

@@ -72,8 +72,10 @@ public class WebhookHttpsIntegrationTests extends AbstractWatcherIntegrationTest
 
     @After
     public void stopWebservice() throws Exception {
+        if (webServer != null) {
             webServer.close();
         }
+    }
 
     public void testHttps() throws Exception {
         webServer.enqueue(new MockResponse().setResponseCode(200).setBody("body"));