libobs: Allow sending NULL to obs_encoder_set_video/audio()

There is currently no way to clear a video_t or audio_t object from an
encoder once applied. `audio_t`/`video_t` objects can be destructed at
any time, and it is dangerous to prevent these object references from
even being cleared.

This does not fix the issue where destroying an audio/video object does
not clear the reference from all subscribed encoders.