Squelch warnings if css: false

[Rich-Harris]

Typically I'll generate two bundles — one for my server app and one for my client app. The client app is responsible for generating CSS (which it passes to rollup-plugin-svelte for concatenation and writing out to a separate .css file), the server app can happily ignore it altogether.

It's annoying if the Rollup logs show 'unused CSS selector' warnings for both bundles.