Fix audio processing for streams with missing audio_bitrate

[B3QL]

Set default audio_bitrate to None.

Similar to #1781 if the file is footage from Hikvision CCTV the bitrate is not available to read even if the audio stream is present.

Sample.mp4

• I have provided code that clearly demonstrates the bug and that only works correctly when applying this fix
• I have added suitable tests demonstrating a fixed bug or new/changed feature to the test suite in tests/
• I have properly documented new or changed features in the documentation or in the docstrings
• I have properly explained unusual or unexpected code in the comments around it

[B3QL]

@mondeja can you take a look? thanks!

[coveralls]

Coverage increased (+0.005%) to 84.87% when pulling 9a79d75 on TraseeHQ:b/hikvision-missing-audio-bitrate into df84762 on Zulko:master.

[mondeja]

Great, thank you!