Empty CSS sourcemaps created in a production build

Created by: viankakrisna

a follow up to https://github.com/facebookincubator/create-react-app/pull/2013#issuecomment-296795003

It's confusing that CRA created an empty .map for css when running npm run build. I thought that it should generate a proper .map file on minify so i've turned the option on that pr. But turns out that css source map is quite problematic (for development) based on https://github.com/facebookincubator/create-react-app/pull/591

So the question right now is whether we should find out why it creates an empty and confusing .map for css file on production, or just turn it on? I don't think it would affect production perf though. http://stackoverflow.com/questions/23848364/performance-impact-of-using-css-javascript-source-maps-in-production and maybe would be beneficial if there is problem with production css.