handle case where exports is null

mustache.js

@@ -6,7 +6,7 @@
 /*global define: false*/
 
 (function (root, factory) {
-  if (typeof exports === "object") {
+  if (typeof exports === "object" && exports) {
     module.exports = factory; // CommonJS
   } else if (typeof define === "function" && define.amd) {
     define(factory); // AMD