Fix: Deprecation notice on PHP 8.1+ (#7)

Author: bpolaszek

src/Renderer/NativeRenderer.php

@@ -37,7 +37,7 @@ public function render(QueryString $queryString): string
     {
         return http_build_query(
             $queryString->getParams(),
-            null,
+            '',
             $this->separator ?? ini_get('arg_separator.output'),
             $this->encoding
         );