fix: more exhaustive check during SvelteMap.set in deriveds

[paoloricciuti]

Closes #13947 ...i'm not sure this is the right fix and i'm not sure of the performance implications. The problem is that basically just checking for null and !null is not enough. In this case both have a reaction because of the second variable read but not every reaction in s are in version.

This seems to fix the issue and tests are green but i have the feeling this could overfire the increase of version.

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[changeset-bot]

🦋 Changeset detected

Latest commit: 59435ac

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

This PR includes changesets to release 1 package

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svelte	Patch

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[pkg-pr-new]

pnpm add https://pkg.pr.new/svelte@13951

commit: 59435ac