fix: show :then block for null value

[dummdidumm]

fixes #14439

This bug was introduced in #13642 because setting the input to null means the equality check ("is the input different") fails if you set the value to null

Also fixes #14441 - this bug was present for a long time, and the reason is the same as for the other bug: The equality check always returns "yes this is the same" if the value is undefined initially. The fix is similar; we need to initialize the input to something that can never be equal to whatever value is passed

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[changeset-bot]

🦋 Changeset detected

Latest commit: 81a7698

The changes in this PR will be included in the next version bump.

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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14440-svelte.vercel.app/

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[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@14440