docs: Fix number input incorrect empty or invalid value

[vladshcherbin]

Fix #15811

Either docs or code is incorrect, current value is null and not undefined as written in docs.

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[changeset-bot]

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Latest commit: c727aee

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15889-svelte.vercel.app/

[Rich-Harris]

Thanks, but this change isn't correct — the value of an empty input is undefined, and this says it's null. I'll close this so we can discuss what we think the right behaviour is on #15811

[vladshcherbin]

My bad, I've used a search param as initial value which is null w/o value 🙈