fix relation bug

Author: listepo

lib/serializer.js

@@ -88,6 +88,9 @@ var parseRelations = function (data, relations, options) {
 };
 
 var makeRelation = function (type, id, options) {
+  if (_.isEmpty(id)) {
+    return null;
+  }
   type = caserize(type, options.keyForRelation);
   var data = {
     type: type,