Initialize $_SERVER['QUERY_STRING'] (Codeception#16)

* Initialize $_SERVER['QUERY_STRING']

* Apache/nginx use an empty string when no query string is set, the PHP internal webserver ommits the property. For now, we cast possible null values to an empty string.

src/Codeception/Lib/Connector/Yii2.php

@@ -301,6 +301,7 @@ public function doRequest($request)
         $queryString = parse_url($uri, PHP_URL_QUERY);
         $_SERVER['REQUEST_URI'] = $queryString === null ? $pathString : $pathString . '?' . $queryString;
         $_SERVER['REQUEST_METHOD'] = strtoupper($request->getMethod());
+        $_SERVER['QUERY_STRING'] = (string)$queryString;
 
         parse_str($queryString, $params);
         foreach ($params as $k => $v) {