return null for undefined relationships

closes #38

Author: danivek

lib/JSONAPISerializer.js

@@ -606,7 +606,7 @@ module.exports = class JSONAPISerializer {
   serializeRelationship(rType, rData, rOptions, included, extraData) {
     // No relationship data
     if (rData === undefined) {
-      return undefined;
+      return null;
     }
 
     // Empty relationship data

test/unit/JSONAPISerializer.test.js

@@ -263,9 +263,9 @@ describe('JSONAPISerializer', function() {
       },
     });
 
-    it('should return undefined for an undefined relationship data', function(done) {
+    it('should return null for an undefined relationship data', function(done) {
       const serializedRelationshipData = Serializer.serializeRelationship('articles', undefined);
-      expect(serializedRelationshipData).to.eql(undefined);
+      expect(serializedRelationshipData).to.eql(null);
       done();
     });