Fix for runaway promise

Author: Jeffrey Mclean

lib/index.js

@@ -165,7 +165,7 @@ module.exports = function loader (css, map) {
            * @param {String} result Result (JS Module)
            * @param {Object} map    Source Map
            */
-          return cb(null, `module.exports = ${JSON.stringify(css)}`, map)
+          return cb(null, `module.exports = ${JSON.stringify(css)}`, map) || null
         }
         /**
          * @memberof loader
@@ -175,7 +175,7 @@ module.exports = function loader (css, map) {
          * @param {String} css  Result (Raw Module)
          * @param {Object} map  Source Map
          */
-        return cb(null, css, map)
+        return cb(null, css, map) || null
       })
   }).catch((err) => {
     return err.name === 'CssSyntaxError' ? cb(new SyntaxError(err)) : cb(err)