fix: use lang('HTTP.pageNotFound') on production 404 page

Author: kenjis

system/CodeIgniter.php

@@ -947,7 +947,7 @@ protected function display404errors(PageNotFoundException $e)
         }
 
         throw PageNotFoundException::forPageNotFound(
-            (ENVIRONMENT !== 'production' || ! $this->isWeb()) ? $e->getMessage() : ''
+            (ENVIRONMENT !== 'production' || ! $this->isWeb()) ? $e->getMessage() : null
         );
     }