hook_image_style_flush is now called and passed a path if ImageStyle::flush is called with a path

hook_image_style_flush is now invoked if \Drupal\image\Entity\ImageStyle::flush is called with the $path parameter set. Previously the hook was only called if no path was passed to this method.
Also, the path parameter is now passed along to the hook implementation.

New hook signature:
function hook_image_style_flush($style, $path = NULL)

If you need to maintain the old behaviour of only acting when no path is passed you will need to add an early return to your hook implementation testing for $path not being NULL.

function hook_image_style_flush($style, $path = NULL) {
  if ($path !== NULL) {
    return;
  }
}