Fix rendering partials w/o data

Thanks drobbins (see https://github.com/janl/mustache.js/pull/113)

mustache.js

@@ -139,7 +139,7 @@ var Mustache = function () {
       if (!partials || partials[name] === undefined) {
         throw({message: "unknown_partial '" + name + "'"});
       }
-      if (typeof context[name] != "object") {
+      if (!context || typeof context[name] != "object") {
         return this.render(partials[name], context, partials, true);
       }
       return this.render(partials[name], context[name], partials, true);