css: Don't throw warnings on broken URL in image css

css: Don't throw warnings on broken URL in image css

If a URL can't be loaded, we might end up with a NULL file. Handle that
case properly by creating an invalid image instead and don't crash or
complain to stderr when files are NULL.

This was broken since 0886ade182f1a958024202d5c20248d57fcdb053

A new reftest has been included. We need a reftest instead of a
CSS parser test, because the error only becomes visible when
compute()ing the actual image.

Fixes #4373