[ARTE] --convert-subs output leads to zero byte files

[someziggyman]

Checklist

• I'm reporting a bug unrelated to a specific site
• I've verified that I'm running yt-dlp version 2022.05.18 (update instructions) or later (specify commit)
• I've checked that all provided URLs are playable in a browser with the same IP and same login details
• I've checked that all URLs and arguments with special characters are properly quoted or escaped
• I've searched the bugtracker for similar issues including closed ones. DO NOT post duplicates
• I've read the guidelines for opening an issue

Description

Downloading subs without conversion works fine.
The output gives files around 70Kb VTT format. However after --convert-subs srt param is given the output are just empty SRT files. No errors in the log

Youtube subs with the same scenario work fine.

./yt-dlp -v --skip-download --write-auto-subs --write-subs --sub-lang "fr,pt,en" --convert-subs "srt" --ffmpeg-location "./yt-dlp/ffmpeg" "https://www.arte.tv/fr/videos/103364-000-A/julian-assange-chronique-d-une-extradition-annoncee/"

Verbose log

[debug] Command-line config: ['-v', '--skip-download', '--write-auto-subs', '--write-subs', '--sub-lang', 'fr,pt,en', '--convert-subs', 'srt', '--ffmpeg-location', './yt-dlp/ffmpeg', 'https://www.arte.tv/fr/videos/103364-000-A/julian-assange-chronique-d-une-extradition-annoncee/']
[debug] Encodings: locale UTF-8, fs utf-8, pref UTF-8, out utf-8, error utf-8, screen utf-8
[debug] yt-dlp version 2022.05.18 [b14d52355] (zip)
[debug] Plugins: ['SamplePluginIE', 'SamplePluginPP']
[debug] Python version 3.9.13 (CPython 64bit) - macOS-12.4-arm64-arm-64bit
[debug] Checking exe version: ./yt-dlp/ffmpeg -bsfs
[debug] Checking exe version: ./yt-dlp/ffprobe -bsfs
[debug] Checking exe version: ./yt-dlp/avprobe -bsfs
[debug] exe versions: ffmpeg 5.0.1 (setts)
[debug] Optional libraries: sqlite3-2.6.0
[debug] Proxy map: {}
[debug] [ArteTV] Extracting URL: https://www.arte.tv/fr/videos/103364-000-A/julian-assange-chronique-d-une-extradition-annoncee/
[ArteTV] 103364-000-A: Downloading JSON metadata
[ArteTV] 103364-000-A: Downloading m3u8 information
[ArteTV] 103364-000-A: Downloading m3u8 information
[ArteTV] 103364-000-A: Downloading m3u8 information
[ArteTV] 103364-000-A: Downloading m3u8 information
[ArteTV] 103364-000-A: Downloading m3u8 information
[ArteTV] 103364-000-A: Downloading m3u8 information
[ArteTV] 103364-000-A: Downloading m3u8 information
[ArteTV] 103364-000-A: Downloading m3u8 information
[debug] Formats sorted by: hasvid, ie_pref, lang, quality, res, fps, hdr:12(7), vcodec:vp9.2(10), acodec, filesize, fs_approx, tbr, vbr, abr, asr, proto, vext, aext, hasaud, source, id
[debug] Downloading subtitles: fr, en
[debug] Default format spec: bestvideo*+bestaudio/best
[info] 103364-000-A: Downloading 1 format(s): VF-STF-2254+VF-STF-program_audio_0-VF
[info] Writing video subtitles to: Julian Assange  - chronique d’une extradition annoncée [103364-000-A].fr.vtt
[debug] Invoking hlsnative downloader on "https://arte-cmafhls.akamaized.net/am/cmaf/103000/103300/103364-000-A/210701113150/medias/103364-000-A_st_VF-MAL.m3u8"
[hlsnative] Downloading m3u8 manifest
[hlsnative] Total fragments: 1
[download] Destination: Julian Assange  - chronique d’une extradition annoncée [103364-000-A].fr.vtt
[download] 100% of 73.11KiB in 00:00
[info] Writing video subtitles to: Julian Assange  - chronique d’une extradition annoncée [103364-000-A].en.vtt
[debug] Invoking hlsnative downloader on "https://arte-cmafhls.akamaized.net/am/cmaf/103000/103300/103364-000-A/210701113150/medias/103364-000-A_st_VO-ANG.m3u8"
[hlsnative] Downloading m3u8 manifest
[hlsnative] Total fragments: 1
[download] Destination: Julian Assange  - chronique d’une extradition annoncée [103364-000-A].en.vtt
[download] 100% of 57.54KiB in 00:00
[SubtitlesConvertor] Converting subtitles
[debug] ffmpeg command line: ./yt-dlp/ffmpeg -y -loglevel repeat+info -i 'file:Julian Assange  - chronique d’une extradition annoncée [103364-000-A].fr.vtt' -f srt -movflags +faststart 'file:Julian Assange  - chronique d’une extradition annoncée [103364-000-A].fr.srt'
[debug] ffmpeg command line: ./yt-dlp/ffmpeg -y -loglevel repeat+info -i 'file:Julian Assange  - chronique d’une extradition annoncée [103364-000-A].en.vtt' -f srt -movflags +faststart 'file:Julian Assange  - chronique d’une extradition annoncée [103364-000-A].en.srt'
Deleting original file Julian Assange  - chronique d’une extradition annoncée [103364-000-A].en.vtt (pass -k to keep)
Deleting original file Julian Assange  - chronique d’une extradition annoncée [103364-000-A].fr.vtt (pass -k to keep)

[pukkandan]

See #3622 (comment) and following conversation

[pukkandan]

Should have been fixed by yt-dlp/FFmpeg-Builds#26