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 0886ade1
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 (closed)
(cherry picked from commit ac8c4245)