[youtube] The case that 'url_encoded_fmt_stream_map' or 'adaptive_fmts' is the empty string is handled accordingly (fixes #4431)
[youtube] Add test case for #4431
Merge remote-tracking branch 'akretz/youtube_fix'
[youtube] Make category optional (#4442)
[youtube] Do not warn if DASH manifest is missing (#4442)
[youtube] Amend test