Cannot parse keys from audio adaptationset

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Shows up as:

```
Traceback (most recent call last):
File "/path/to/Downloads/web-dl/./web-dl.py", line 49, in
main()
File "/path/to/Downloads/web-dl/./web-dl.py", line 35, in main
video_keys, audio_keys = decryptor.extract_keys(args.url())
File "/path/to/Downloads/web-dl/modules/decryptor.py", line 129, in extract_keys
selected_adaptationset_audio_idx = adaptationsets_audio[0][0]
IndexError: list index out of range
```

since `adaptationsets_audio` is empty. The audio adaptationset element doesn't contain "lang" in my case, so the loop in `decrypt.py` passes over it and doesn't pick up the keys.

Here's the offending dash manifest: [stream.mpd.zip](https://github.com/iWas-Coder/web-dl/files/14379498/stream.mpd.zip)

Reproduce the failure with the provided stream.mpd.zip manifest, then inspect modules/decryptor.py around line 129 and the loop in decrypt.py that filters audio adaptation sets. Done means the audio adaptation set is recognized even without a lang attribute and key extraction no longer raises IndexError.

Written by the indexing model from the issue text.