title: Mule MEL check for NullPayload vs null
body: <p>According to the following ticket: <a href="https://www.mulesoft.org/jira/browse/MULE-6427" rel="nofollow">https://www.mulesoft.org/jira/browse/MULE-6427</a> For <code>NullPayload</code> I should be able to use :</p>
  
  <pre><code>&lt;when expression="#[payload == null]"&gt;
  </code></pre>
  
  <p>but that fails. So instead I have to use:</p>
  
  <pre><code>&lt;when expression="#[payload is NullPayload]"&gt;
  </code></pre>
  
  <p>Am I doing something wrong? Or is this the correct way for checking for <code>null</code> or <code>NullPayload</code>?</p>