fix: don't issue a11y warning for video without captions if it has no src

[Rich-Harris]

A <video> without a child <track kind="captions"> element yields an a11y warning. This is unhelpful in the case where, for example, the element in question is being used to show a MediaStream (e.g. the user's webcam).

We could debate whether the warning should be relaxed altogether (not every video needs captions) but I think there's probably value in keeping it and requiring the developer to be explicit about that fact when there's a src. But when there is no src, the warning feels misplaced.

Accompanies #17310

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Playground

pnpm add https://pkg.pr.new/svelte@17311

[paoloricciuti]

I'm gonna approve in case you need a thumb for it but maybe we should wait for an a11y expert for this?

cc @geoffrich

[geoffrich]

Makes sense to me