fix: omit unnecessary nullish coallescing in template expressions

[Ocean-OS]

Currently, Svelte will almost always append a nullish coallescence statement (maybeNullish ?? alternate) to any interpolations in a template expression, regardless of whether there already is one. This changes that, so that nullish coallescence to a non-null literal will stay the same, and in stuff ?? null cases, null is replaced with an empty string.

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[changeset-bot]

🦋 Changeset detected

Latest commit: e58a65a

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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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-15056-svelte.vercel.app/

this is an automated message

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15056

[paoloricciuti]

Can you also add a test for this?