fix: empty CSS file was output when only .css?url is used

[sapphi-red]

Description

When import u from './foo.css?url' is used and import './bar.css' is not used, an empty CSS file was output.

fixes #15843

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines, especially the Pull Request Guidelines.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Update the corresponding documentation if needed.
• Ideally, include relevant tests that fail without this PR but pass with it.

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[sapphi-red]

If a file contains __VITE_CSS_URL__ (!s === false) and doesn't have side effect CSS imports (e.g. import './foo.css') (!chunkCSS === true), the if-else block below should not run and if (s) block should run.

[patak-dev]

/ecosystem-ci run

[vite-ecosystem-ci]

📝 Ran ecosystem CI on 46a7199: Open

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