fix(css): inject source content conditionally

[antfu]

Description

As how we handle it in JS:

vite/packages/vite/src/node/server/transformRequest.ts

Line 271 in c4aa28f

if (map.mappings && !map.sourcesContent) {

We should also guard injectSourcesContent in CSS source map for virtual CSS modules

Additional context

unocss/unocss#2113 (comment)

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the PR Title Convention.
• 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).
• Ideally, include relevant tests that fail without this PR but pass with it.

[stackblitz]

Run & review this pull request in StackBlitz Codeflow.