Fix toObject again

Author: ericingram

lib/schema.js

@@ -1418,8 +1418,10 @@
       var keys = Object.keys(this.__data);
       for (var i = 0; i < keys.length; i++) {
         var key = keys[i];
-        data = data || {};
-        data[key] = this.__data[key];
+        if (this.__data[key]) {
+          data = data || {};
+          data[key] = this.__data[key];
+        }
       }
     }
     return data;