don't die on null values

mustache.js

@@ -194,7 +194,7 @@ var Mustache = function() {
       Does away with nasty characters
     */
     escape: function(s) {
-      return s.toString().replace(/[&"<>\\]/g, function(s) {
+      return (s ? s.toString() : "").replace(/[&"<>\\]/g, function(s) {
         switch(s) {
           case "&": return "&amp;";
           case "\\": return "\\\\";;