Don't add env variable when template is empty (e.g. on error page)

program/include/rcube_template.php

@@ -342,6 +342,7 @@ class rcube_template extends rcube_html_page
             array_unshift($this->js_commands, array('hide_message', $unlock));
         }
 
+        if (!empty($this->script_files))
           $this->set_env('request_token', $this->app->get_request_token());
 
         // write all env variables to client