Fixed error when loader processes file with no theme vars

Author: samuelmaddock

lib/loader.js

@@ -61,6 +61,9 @@ module.exports = function (source, map) {
             return null;
         });
     }).then(function (prevResult) {
+        // no theme css processing needed
+        if (!prevResult) { return; }
+
         const content = prevResult.themeCss;
 
         const options = loaderUtils.getOptions(loader) || {};