fix: ensure undefined class still applies scoping class, if necessary

[dummdidumm]

fixes #15635

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🦋 Changeset detected

Latest commit: 43cca07

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

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svelte	Patch

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15643-svelte.vercel.app/

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Playground

pnpm add https://pkg.pr.new/svelte@15643