opus: sample rate is always 48kHz
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).
opus: changelog/thanks/docs (closes #377, #381)