sass-loader does not invoke custom importers

This happens because of wrong return value.
I have fixed it in my PR according to node-sass docs: https://github.com/sass/node-sass#importer--v200---experimental

> importer can be an array of functions, which will be called by LibSass in the order of their occurrence in array. This helps user specify special importer for particular kind of path (filesystem, http). If an importer does not want to handle a particular path, it should return null

https://github.com/egoist/rollup-plugin-postcss/pull/201