Cannot generate image URL for images style "_original image_" and "_empty image_"

A quick and dirty fix:

\Drupal\responsive_image_preload\PreloadGenerator::generatePreloadsForDelta() #144:

case 'image_style':
              $image_style_name = $image_style_mapping['image_mapping'];
              /** @var \Drupal\image\ImageStyleInterface $image_style */
              $image_style = $image_style_storage->load($image_style_name);
              $url = $image_style ? $image_style->buildUrl($file->getFileUri()) : $file->getFileUri();
              $image_url = $this->fileUrlGenerator->generateString($url);

I have uploaded the patch

Wow, I'm sorry. This issue never reached my inbox!

I'll try to find some time to get this evaluated. My gut is saying we skip the empty image (since it's an inline data: source -- preloading won't make any difference here) and treat the original image as the patch in #3 suggests.

This also needs some extended test coverage to trigger a failure in Drupal CI before a patch can be committed to fix it.