How to check null conditions before adding headers?

[sudhasusarita]

I would want to add headers only when the request.user.agent is not null.
How to achieve it?

<AssignMessage continueOnError="false" enabled="true" name="add-headers-1">
  <Add>
    <Headers>
      <Header name="user-agent">{request.user.agent}</Header>
    </Headers>
  </Add>
  <AssignTo createNew="false" transport="http" type="request"/>
</AssignMessage>

 @dchiesa1