fix: treat undefined and null the same for the initial input value

[dummdidumm]

Fixes #14558

Any unforseen side effects that could come from this? (didn't add a test yet because I'm in a rush and want to have that question answered first)

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[changeset-bot]

🦋 Changeset detected

Latest commit: 7db8e13

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

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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14562-svelte.vercel.app/

this is an automated message

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@14562

[trueadm]

A test case would be wonderful

[dummdidumm]

Any drawbacks you see to this approach?

[trueadm]

Any drawbacks you see to this approach?

Nothing stands out for me. So it's probably fine. I just thought a test case to cover this regression would be cool.