opus: remove image from test file
opus: remove image from test file

The encoder that produced this file for some reason included an empty image as
the cover art and was confusing the tests.

I left the null check in place to deal with this situation in the future. I
think returning None is better than returning the empty string (which is of
course not a valid image).

Original: 2e23e8204135353f00917f17b08a8f5344fd9e8a@6857267

beetbox/mediafile