[FIX] Fix missing X-TIMESTAMP-MAP header in WebVTT when no subtitles found

[dRaniwal]

Problem

I noticed that when running CCExtractor with the --timestamp-map flag on a video file that has no subtitles, the resulting WebVTT file was just empty with only "WEBVTT" at the top. The X-TIMESTAMP-MAP header was completely missing, which breaks HLS streaming compatibility.

What I changed

• Updated write_webvtt_header() so it can write to a specific output file
• Added a fallback that writes a default timestamp map (X-TIMESTAMP-MAP=MPEGTS:0,LOCAL:00:00:00.000) when there's no timing info available
• Made sure WebVTT files get their header written even when there are no subtitles by handling this in write_subtitle_file_footer()
• Fixed a cleanup issue where encoders weren't being properly closed if no subtitles were found

Fixes #1743

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Testing

Ran CCExtractor on a .ts file with no subtitles using the --timestamp-map flag. Before this fix, the output was just "WEBVTT". Now it correctly outputs:

[ccextractor-bot]

CCExtractor CI platform finished running the test files on linux. Below is a summary of the test results, when compared to test for commit 42d7509...:

Report Name	Tests Passed
Broken	13/13
CEA-708	14/14
DVB	7/7
DVD	3/3
DVR-MS	2/2
General	27/27
Hardsubx	1/1
Hauppage	3/3
MP4	3/3
NoCC	10/10
Options	86/86
Teletext	21/21
WTV	13/13
XDS	34/34

Congratulations: Merging this PR would fix the following tests:

• ccextractor --hardsubx 1a0302f7fd..., Last passed: Never

All tests passing on the master branch were passed completely.

Check the result page for more info.

[dRaniwal]

Hi! The Windows CI failure is unrelated to my changes - it's failing due to a vcpkg download issue with libxml2 from GitLab:
Download failed, halting portfile. error: building libxml2:x64-windows-static failed with: BUILD_FAILED

I've noticed other PRs are hitting the same issue. This seems to be a transient network/infrastructure problem with the external dependency mirror.

Would it be possible to re-run the CI, or could this be merged if the Linux/macOS builds pass? Happy to make any actual code changes if needed!

[cfsmp3]

Would it be possible to re-run the CI, or could this be merged if the Linux/macOS builds pass? Happy to make any actual code changes if needed!

I've restarted the tests.

[steel-bucket]

The Windows CI malfunction is currently being fixed. The vcpkg problem has been fixed(credit to @DhanushVarma-2), but the Rust Build isn't working for the latest changes as there were some uncaught errors that were merged. We're working on it.