fix(css): inject source content conditionally (#12449)

Author: antfu

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

@@ -377,7 +377,9 @@ export function cssPostPlugin(config: ResolvedConfig): Plugin {
         const getContentWithSourcemap = async (content: string) => {
           if (config.css?.devSourcemap) {
             const sourcemap = this.getCombinedSourcemap()
-            await injectSourcesContent(sourcemap, cleanUrl(id), config.logger)
+            if (sourcemap.mappings && !sourcemap.sourcesContent) {
+              await injectSourcesContent(sourcemap, cleanUrl(id), config.logger)
+            }
             return getCodeWithSourcemap('css', content, sourcemap)
           }
           return content