title: Jackson Mapper serialize empty object instead of null
body: <p>Say I have classes Foo</p>
  
  <pre><code>public class Foo {
      private Bar bar;
  }
  </code></pre>
  
  <p>and Bar</p>
  
  <pre><code>public class Bar {
      private String fizz;
      private String bang;
  }
  </code></pre>
  
  <p><em>EDIT</em>: For clarification I do not own <code>Foo</code> and <code>Bar</code> and cannot alter these classes.</p>
  
  <p>If I want to serialize an empty object of type Foo, it's member, which is of type Bar, will be returned as null.</p>
  
  <pre><code>String json = objectMapper.writeValueAsString(new Foo()); // "{"bar" : null}"
  </code></pre>
  
  <p>Is there any way I can get the object mapper to serialize an empty Bar object without having to instantiate a new instance of Bar and then adding it to a new instance of Foo?</p>
  
  <pre><code>String json = objectMapper.writeValueAsString(new Foo()) // "{bar": {"fizz" : null, "bang" : null } }"
  </code></pre>