fix(css): return deps if have no postcss plugins

[bluwy]

Description

After we preprocess but there's no postcss plugins to apply, we should still return the deps it relies on.

The deps could be populated at

vite/packages/vite/src/node/plugins/css.ts

Lines 910 to 917 in 42b3434

	if (preprocessResult.deps) {
	preprocessResult.deps.forEach((dep) => {
	// sometimes sass registers the file itself as a dep
	if (normalizePath(dep) !== normalizePath(opts.filename)) {
	deps.add(dep)
	}
	})
	}

Additional context

Found this while investigating withastro/astro#7035

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Pull Request Guidelines and follow the PR Title Convention.
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[patak-dev]

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[vite-ecosystem-ci]

📝 Ran ecosystem CI: Open

suite	result
astro	✅ success
histoire	✅ success
iles	✅ success
ladle	✅ success
laravel	✅ success
marko	✅ success
nuxt	❌ failure
nx	✅ success
previewjs	✅ success
qwik	✅ success
rakkas	✅ success
sveltekit	✅ success
unocss	✅ success
vite-plugin-pwa	✅ success
vite-plugin-ssr	✅ success
vite-plugin-react	✅ success
vite-plugin-react-pages	✅ success
vite-plugin-react-swc	✅ success
vite-plugin-svelte	✅ success
vite-plugin-vue	✅ success
vite-setup-catalogue	✅ success
vitepress	✅ success
vitest	✅ success