Perhaps this blog post could explain a way:
http://blog.lilybiri.com/where-is-null
Of course, you need a trigger to launch the advanced conditional action. Are you using the Submit button and/or a shortcut key for the TEB? And I suppose it is not validated. Then the even can be the On Success for the TEB.
- create the button and set it to invisible (or disable it)
- create the empty variable v_null
- create a conditional action that checks the associated variable to the TEB, if it is equal to v_null, nothing has to happen (No action), ELSE show (or enable) the button
Lilybiri