title: Flask-RESTful - don't return object property instead of returning null
body: <p>Let's say I've got a clients table with <strong>id</strong>, <strong>name</strong> and <strong>email</strong> fields. An <strong>email</strong> field is <strong>optional</strong>. </p>
  
  <p>The code looks like this:</p>
  
  <pre><code>client_fields = {
     'id' : fields.String,
     'name' : fields.String,
     'email' : fields.String
  }
  </code></pre>
  
  <p>And for displaying:</p>
  
  <pre><code>class ClientList(Resource):
      @marshal_with(client_fields)
      def get(self):
         return model.Client.query.all()
  </code></pre>
  
  <p>When email is not provided, API returns JSON like this:</p>
  
  <pre><code>{
     "id": "1",
     "name": "John Doe",
     "email": null
  }
  </code></pre>
  
  <p>But instead I want it to return this object:</p>
  
  <pre><code>{
     "id": "1",
     "name": "John Doe"
  }
  </code></pre>
  
  <p>Which basically means that instead of a property with null value I want it to <strong>return no such property at all</strong>. Is there a way to achieve that?</p>