title: Jackson JSON generator creates null JSON values for missing objects
body: <p>I've started using Jackson as a JSON generator, as an alternative to google GSON.  I've run into an issue where Jackson is generating object: null if the object is indeed null.  GSON on the other hand generates NO entry in JSON, which is the behavior I want.  Is there a way to stop Jackson from generating null object/value when an object is missing?</p>
  
  <p>Jackson</p>
  
  <pre><code>        ObjectMapper mapper = new ObjectMapper();
      StringWriter sw = new StringWriter();
      mapper.writeValue(sw, some_complex_object);
      String jackson = sw.getBuffer().toString();
  
      System.out.println("********************* START JACKSON JSON ****************************");
      System.out.println(jackson);
      System.out.println("********************* END JACKSON JSON ****************************");
  </code></pre>
  
  <p>generates this:</p>
  
  <p>{"eatwithrustyspoon":{<strong>"urlList":null</strong>,"device":"iPad","os":"iPhone OS","peer_id":</p>
  
  <p>and GSON looks like this:</p>
  
  <pre><code>        Gson gson = new Gson();
      String json = gson.toJson(some_complex_object);
  
      System.out.println("********************* START GSON JSON ****************************");
      System.out.println(json);
      System.out.println("********************* END GSON JSON ****************************");
  </code></pre>
  
  <p>and it generates this (which is what I want - note that "urlList":null was not generated) :</p>
  
  <p>{"eatwithrustyspoon":{"device":"iPad","os":"iPhone OS","peer_id"</p>