fix members request with empty query.

members request does not require query and is actually broken right now if you provide an empty query
https://gitlab.com/gitlab-com/support-forum/issues/2878

Author: Artistan

lib/Gitlab/Api/Projects.php

@@ -247,9 +247,8 @@ public function cancelPipeline($project_id, $pipeline_id)
      */
     public function members($project_id, $username_query = null)
     {
-        return $this->get($this->getProjectPath($project_id, 'members'), array(
-            'query' => $username_query
-        ));
+        $params = $username_query??[];
+        return $this->get($this->getProjectPath($project_id, 'members'), $params);
     }
 
     /**