fix: avoid unnecessary read version increments

[dummdidumm]

Fixes #15262

Apologies @trueadm , you were right in #15694 that the check was not redundant, my suggestion was wrong. reaction cannot be null, so when previous_reaction is null the check was now true when it should've been false. Therefore reinstating this check.

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15777-svelte.vercel.app/

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Playground

pnpm add https://pkg.pr.new/svelte@15777