title: WSO2 Filter Proxy Boolean Expression evaluator issue
body: <p>in my proxy i'm using a filter mediator. My target is to activate a sequence if some conditions on some properties are verified:</p>
  
  <p>codice = 0
  idElementoCross is different from null or empty string or if it exists
  tipoElementoCross is different from null or empty string or if it exists</p>
  
  <p>i wrote this condition:</p>
  
  <pre><code> &lt;sequence&gt;
       &lt;property name="codice" expression="//codice"&gt;&lt;/property&gt;
       &lt;property name="idElementoCross" expression="//idElementoCross"&gt;&lt;/property&gt;
       &lt;property name="tipoElementoCross" expression="//tipoElementoCross"&gt;&lt;/property&gt;
       &lt;filter xpath="boolean(fn:get-property('codice')=0 and fn:get-property('tipoRisposta')='worker' and fn:get-property('tipoElementoCross')!='null' and fn:get-property('idElementoCross')!='null' and fn:get-property('tipoElementoCross')!='' and fn:get-property('idElementoCross')!='' and fn:get-property('tipoElementoCross') and fn:get-property('idElementoCross'))"&gt;
                   &lt;then&gt; ...
  </code></pre>
  
  <p>but in my log i see that esb enter the sequence even if  idElementoCross and tipoElementoCross are empty.
  Im using WSO2 ESB 4.8.1.</p>
  
  <p>What am i missing?</p>