feat: simpler string normalization

[Rich-Harris]

It turns out that text.nodeValue = undefined (or null) results in text.nodeValue === '', and in all the places we currently use stringify implicit coercion happens anyway, so:

• we don't need to use $.stringify when updating text nodes directly (unless it's inside a template literal, where we can just use ?? '')
• we don't need to use it when setting input values (again, just use ?? '')

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Latest commit: 2971e00

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