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 something else, and then changed to undefined, the input value was "undefined" 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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