title: Mule expression language boolean
body: <p>I have an enricher which invokes a method and returns true or false. However, when I test the condition it doesn't trigger the action. The documentation says that testing with keyword 'empty' returns true if the value in the expression returns false. When the checkExisting returns false, I expect the Action to be triggered but that does not happen.  What is the problem? </p>
  
  <pre><code>&lt;enricher target="#[variable:myVal]"&gt;
    &lt;invoke object-ref="myBean" method="checkExisting" methodArguments="#
                                                                 [payload]"/&gt;
  &lt;/enricher&gt;
  
  I check the result like this
     &lt;choice&gt;
       &lt;when expression="#[myVal==empty]" &gt;
        ... Action to take..
  </code></pre>