Fix backend to None while ffmpeg is unavailable.

[pengzhendong]

Before:

After:

[pzelasko]

Wait, that's weird. This code is guarded by if torchaudio_ffmpeg_backend_available(): so it shouldn't have entered this branch at all. Perhaps you could move this extra logic inside torchaudio_ffmpeg_backend_available instead? Looks like the check there may be insufficient.

[pengzhendong]

If it is moved to torchaudio_ffmpeg_backend_available, Lhotse will utilize torchaudio.StreamReader to read the audio instead of torchaudio.info.

ImportError: Failed to initialize the FFmpeg extension. Tried versions: ['6', '5', '4', '']. Enabling DEBUG logging will provide more details about the error.

[pzelasko]

I'm still not quite sure what's going on but your change seems unlikely to break anything and the backward compatibility tests are passing. LGTM, thanks!