Ensure that validation passes if image missing

When we simply validate against the image we can run into a
race condition where the image is temporarily unavailable due to virus
check, or on preview and development environments where we don't copy
the assets.

If the `ImageData` record is invalid for this reason, we weren't able to
copy over this association when republishing the document. Therefore we
need to allow the image to validate based on the presence of the foreign
key, as well as the presence of an Image.

This doesn't break the previous 'featuring without an image' behaviour,
as we still check to see whether an `ImageData` record is successfully
created in the first place.

Ticket: https://www.pivotaltracker.com/story/show/45879539