Improve Combobox Input value

[RobinMalfait]

This PR improves upon this PR: #1181

In that PR we ignored syncing when your value was set to undefined, that means that if you "reset" your value to undefined that the old value was still present in the Input. To fix this, we will always sync (unless we don't have an input field).
This means that your displayValue should always return something. It could happen that you have to write this:

(React example)

<Combobox.Input displayValue={person => person?.name} />

This would have resulted in "undefined" in the input, so we will also make sure that null and undefined values are converted to an empty string (''). It's important that we use ?? '' and not || '', otherwise values like 0 would be converted to an empty string as well.

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headlessui-react – ./packages/playground-react

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headlessui-vue – ./packages/playground-vue

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✅ Preview: https://headlessui-vue-git-reset-input-on-value-change-tailwindlabs.vercel.app