title: How to show null value in JSON in FOS Rest Bundle with JMS Serializer?
body: <p>I had a read through this : <a href="https://github.com/schmittjoh/serializer/issues/77">https://github.com/schmittjoh/serializer/issues/77</a> but did not find any way to serialize null values in JSON for FOS Rest bundle with JMS serializer (meaning just show the key of the Doctrine object even if its null).</p>
  
  <p>I am using the following config in composer.json</p>
  
  <pre><code>"jms/serializer-bundle": "0.12.*@dev",
  "friendsofsymfony/rest-bundle": "0.13.*@dev",
  </code></pre>
  
  <p>The JMS serializer config</p>
  
  <pre><code>#jms-serializer
  jms_serializer:
   visitors:
      json:
          options: 0 # json_encode options bitmask
          serialize_null: true
  </code></pre>
  
  <p>Or the FOS Rest bunde config</p>
  
  <pre><code>fos_rest:
  view:
      serialize_null: true
  </code></pre>
  
  <p>Does not work. I'm not using a view I'm "view_response_listener: 'force'" so if a solution from the config can be provided it would help, thanks.</p>