Not null condition on group in BI Publisher report

I have an unchangeable (I mean this is a standard report I cannot make changes to) XML like

<LIST_G1>

     <G1>

          <Field1>

          <LIST_G2>

               <G2>

                    <Field2>

                    <LIST_G3>

                         <G3>

                              <Field3>

                         </G3>

                    </LIST_G3>

               </G2>

          </LIST_G2>

     </G1>

</LIST_G1>

Now I want to print a table with this nested structure and it should look like

<Field1>  <Field2> <Field3>

This will work if I add this

<?for-each:G3?><?../../../../FIELD1?>   <?../../FIELD2?>   <?FIELD3?><?end for-each?>

So far so good, but it may happen that G3 is empty. In that case my XML is like

<LIST_G1>

     <G1>

          <Field1>

          <LIST_G2>

               <G2>

                    <Field2>

                    </LIST_G3>