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

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.
• If this PR changes code within packages/svelte/src, add a changeset (npx changeset).

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[changeset-bot]

🦋 Changeset detected

Latest commit: 0029c5d

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
svelte	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[github-actions]

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