Created by: gaearon
Fixes an issue I introduced in https://github.com/facebookincubator/create-react-app/pull/2261. Travis didn't catch it because of https://github.com/facebookincubator/create-react-app/issues/2263.
In particular, if it is empty, ''.split() gives us [''] and then webpack complains about empty value at the end when validating the config. Which is neat!