Returns None if desc is NULL
Returns None if desc is NULL

avcodec_descriptor_get() returns NULL when no descriptor exists.
https://ffmpeg.org/doxygen/3.2/group__lavc__misc.html#gac09f8ddc2d4b36c5a85c6befba0d0888
pupil-labs/PyAV,pupil-labs/PyAV,pupil-labs/PyAV,PyAV-Org/PyAV,pupil-labs/PyAV,mikeboers/PyAV,mikeboers/PyAV,PyAV-Org/PyAV