title: Null condition in qliksense expression
body: <p>i have this expression </p>
  
  <pre><code>sum( {&lt;code={'12','13,'16','90'}, type={'abc','def','xyz'}, type2={'general'}&gt;} net_pre)
  -
  sum( {&lt;code={'12','13,'16','90'}, type={'abc','def','xyz'}, type2={'general'}&gt;} tax_prem)
  </code></pre>
  
  <p>in that i want to add condition </p>
  
  <pre><code>A.Code Is Null Or (A.Code Is Not Null And A.MClass Is Null)
  </code></pre>
  
  <p>Currenly i did not not tried anything about that and dont know how to add this condition in expression </p>
  
  <p>how i modify expression </p>
  
  <p>any help ?</p>