null/ blank value check for a process variable (pv)

This code for check the process variable "gap" Boolean is not working.

My sample rule is:

if(
rule!APN_isBlank(ri!gap),
if(
like(ri!text,"*GAP*"),
ri!gap=true(),
ri!gap=false()),
ri!gap)

I need to check if the pv- gap (boolean) is null or empty, then I have to populate the gap as with checking on the like function

Else I need to just display the gap (true/ false) as it is which is not null.

Problem: Here when I give null/ empty value for "gap", then the true part is executed. It says simply false, even when the like function gives true value (text contains words "gap"). Please help me to execute this scenario.