perf(css): skip style.css extraction if code-split css

[bluwy]

Description

If we're code-splitting CSS, there's a portion of the code that was still extracting combined CSS chunks to emit the non-codesplit style.css chunk, which would always be empty.

The extraction was happening from

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

Line 887 in bd540d5

css += chunkCSSMap.get(chunk.preliminaryFileName) ?? ''

However the chunkCSSMap is only populated cssCodeSplit is disabled

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

Lines 832 to 838 in bd540d5

	} else {
	// resolve public URL from CSS paths, we need to use absolute paths
	chunkCSS = resolveAssetUrlsInCss(chunkCSS, cssBundleName)
	// finalizeCss is called for the aggregated chunk in generateBundle
	chunkCSSMap.set(chunk.fileName, chunkCSS)
	}

This PR updates so that the extracting of combined CSS chunks only happens if cssCodeSplit is disabled.