Conditional expressions in XML Business Rules

The XML below has an expression that works.

What if I want to change this expression to have a conditional: the result must be blank if person.mobile_phone is blank.

Thanks 

<update_person>
  <person>
    <person_id>@person_id</person_id>
    <alt_email_address>@expression[person.mobile_phone + "@textmarketer.biz"]</alt_email_address>
    <insert_update is_initialized="False" />
  </person>
</update_person>