Watcher: Prevent question mark in HttpClient with empty params (elastic/x-pack-elasticsearch#4206)

The HTTPClient in watcher always appended a question mark at the end of
an URL, regardless if parameters were used or not. This commit adds a
check to only pass valid parameters to the URI construction.

Original commit: elastic/x-pack-elasticsearch@184f8f441c

plugin/watcher/src/main/java/org/elasticsearch/xpack/watcher/common/http/HttpClient.java

@@ -241,8 +241,9 @@ public class HttpClient extends AbstractComponent {
         try {
             List<NameValuePair> qparams = new ArrayList<>(request.params.size());
             request.params.forEach((k, v) -> qparams.add(new BasicNameValuePair(k, v)));
+            String format = URLEncodedUtils.format(qparams, "UTF-8");
             URI uri = URIUtils.createURI(request.scheme.scheme(), request.host, request.port, request.path,
-                    URLEncodedUtils.format(qparams, "UTF-8"), null);
+                    Strings.isNullOrEmpty(format) ? null : format, null);
 
             return uri;
         } catch (URISyntaxException e) {

plugin/watcher/src/test/java/org/elasticsearch/xpack/watcher/common/http/HttpClientTests.java

@@ -595,4 +595,15 @@ public class HttpClientTests extends ESTestCase {
         assertThat(webServer.requests().get(0).getHeader(HttpHeaders.CONTENT_TYPE), is(XContentType.JSON.mediaType()));
         assertThat(webServer.requests().get(0).getBody(), is(body));
     }
+
+    public void testThatUrlDoesNotContainQuestionMarkAtTheEnd() throws Exception {
+        webServer.enqueue(new MockResponse().setResponseCode(200).setBody("whatever"));
+
+        HttpRequest request = HttpRequest.builder("localhost", webServer.getPort())
+                        .path("foo")
+                        .build();
+        httpClient.execute(request);
+        assertThat(webServer.requests(), hasSize(1));
+        assertThat(webServer.requests().get(0).getUri().getRawPath(), is("/foo"));
+    }
 }