Don't add an empty stylesheet to the bundle if there are no CSS files

Some users don't use CSS stylesheets in their applications, see for instance [0] and [1]. Meteor shouldn't force them to include an empty stylesheet.

[0] https://groups.google.com/forum/#!topic/meteor-talk/iqhycx-Fg5I 
[1] http://pem-musing.blogspot.fr/2014/07/do-we-need-html-and-css-when-we-got.html