I don't understand what you mean, %topic.part.1 wont have a value when you first initiate the $?. You could accomplish this by seeing if the var is $true.
I've used $input() instead of $?.
Code:
$iif(%topic.part.1,$input(Topic part #1:,e,Input Request,%topic.part.1),$input(Topic part #1:,e,Input Request))