fix: consistently set value to blank string when value attribute is undefined

[theodorejb]

Previously there was an inconsistency where a value attribute (correctly) wasn't set if initially undefined (resulting in the input value being a blank string), but if the value attribute was first set to a non-nullish value, and then changed to undefined, then the string "undefined" would appear as the input value, rather than the expected empty string.

Fixes #4467

[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-15057-svelte.vercel.app/

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[github-actions]

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pnpm add https://pkg.pr.new/svelte@15057

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[theodorejb]

@dummdidumm Can you review this?

We ran into bugs from this where an input value was set from an optional URL param. E.g:

<input name="address" value={params.address} />

This appears to work great at first, since when the parameter isn't set initially the input is blank, and when the parameter is added it is displayed as expected in the input. But if a subsequent navigation removes the parameter after it had been set, the input unexpectedly shows "undefined".

[dummdidumm]

Looks good, thanks! Add a changeset and we're good to merge 👍