[webvtt] Don't parse single fragment files

[seproDev]

IMPORTANT: PRs without the template will be CLOSED

Description of your pull request and other information

If there is only a single fragment, it is just a full file and thus does not need to be parsed.

Fixes the symptoms of #5804
The webvtt parser should still be made more lenient.

Template

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• Core bug fix/improvement
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