fix: 400 bad request api issue with graphql when variables are not passed

Author: svishalgarg

lib/HttpRequestGraphQL.php

@@ -49,6 +49,8 @@ protected static function prepareRequest($httpHeaders = array(), $data = array()
 
  if (is_array($variables)) {
  self::$postDataGraphQL = json_encode(['query' => $data, 'variables' => $variables]);
+ } else {
+ self::$postDataGraphQL = json_encode(['query' => $data]);
  }
  }