Fix error when trying to delete image field set to None

The signal receiver ``delete_image_files`` fails when attempting to delete
an image field that is ``None``. This raises a ``ThumbnailError`` rather than
handling it gracefully.

I added a test for that and basically ignore the exception as I can't think of
a case where this should be handeled differently. We are trying to delete the
image anyway, so this shouldn't matter.