Fast404 MUST exclude paths

Problem/Motivation

Drupal now includes inbuilt Fast404 support controlled by a hidden system setting. This includes an option to exclude some paths from triggering a 404: this defaults to '/\/(?:styles|imagecache)\//' (excluding image cache generated images from the fast 404). Should you not want to exclude these paths and set the setting to empty, the Fast404 system is disabled for all paths.

Steps to reproduce

In system.performance.yml set the exclude_paths option to empty and import config. Visit a path that should trigger a Fast404 response: the standard Drupal 404 page will display.

Proposed resolution

The code in Fast404ExceptionHtmlSubscriber.php::on404() includes the line:

if ($config->get('fast_404.enabled') && $exclude_paths && !preg_match($exclude_paths, $request->getPathInfo())) {

This means that the $exclude_paths variable must be set - from $config->get('fast_404.exclude_paths'); - before the code that checks whether the current path is one that should trigger a Fast404: if it isn't set, no code runs and the standard 404 is returned.

Is there some reason I'm not aware of that we shouldn't generate Fast404s for image cache image styles?