Style derivatives are skipped because file_exists check returns false positive

This patch was very useful when combined with ✨ Add possibility to skip flushing all image style derivatives when an image style configuration changes Needs work . It allows to slowly replace old derivatives with the new ones when an image style change is done without the danger of having to remove all them and wait hours for the images to be re-created.

If the Drupal Core issue is merged at some point, it would be great to convert this in a "Feature Request" and maybe add a setting to control the behaviour.