fix: handle None quality in video/audio stream comparison

[xiaoyuyu6420]

Problem

VideoDownloadURLDataDetecter.detect_best_streams() crashes with AttributeError: 'NoneType' object has no attribute 'value' when a stream in the DASH response has video_quality / audio_quality set to None.

Both comparator functions (video_stream_cmp / audio_stream_cmp) call .value directly on the quality enums:

• video_stream_cmp: if s1.video_quality.value != s2.video_quality.value → s1.video_quality.value - s2.video_quality.value
• audio_stream_cmp: return s1.audio_quality.value - s2.audio_quality.value

Reproduced on PyPI bilibili-api-python==17.4.2 (detect_best_streams on line 2636), breaking video download/resolution detection for some videos.

Fix

Guard both comparators: if either stream's quality is None, return 0 before the Dolby/HDR/Hi-Res priority checks.

if s1.video_quality is None or s2.video_quality is None:
    return 0

if s1.audio_quality is None or s2.audio_quality is None:
    return 0

Verification

A/B tested against the released 17.4.2 source with detect() mocked to return streams carrying None quality:

• unpatched: AttributeError: 'NoneType' object has no attribute 'value' (video.py:2636)
• patched: detect_best_streams() returns the stream list normally

No behavior change for streams with valid quality values.