[extractor/tiktok] Fix and improve metadata extraction

[bashonly]

• Fixes metadata extraction from video webpages (_parse_aweme_video_web):

  • creator was failing to be extracted (addresses #6483 (comment))
  • thumbnails were failing to be extracted
  • id extraction was not fatal, and the value always matched from the URL could easily be passed as an argument to _parse_aweme_video_web

• Fixes some potentially fatal metadata extraction from mobile API responses (_parse_aweme_video_app):

  • thumbnails extraction KeyErrored if a Douyin API response was received
  • author_info = aweme_detail.get('author', {}) should be aweme_detail.get('author') or {} since author can be None, etc

• Extracts the secondary user ID as channel_id from both web and app, which will be useful/necessary for a TikTokUserIE fix (it already would be useful for [extractor/tiktok] Fix TikTokUserIE #4996)

• Refactors and attempts to improve readability

• Adds a webpage-only test case, updates tests

Template

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• At least skimmed through contributing guidelines including yt-dlp coding conventions
• Searched the bugtracker for similar pull requests
• Checked the code with flake8 and ran relevant tests

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• New extractor (Piracy websites will not be accepted)
• Core bug fix/improvement
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