Empty CSS sourcemaps created in a production build

[viankakrisna]

a follow up to #2013 (comment)

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 #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.