SugarBPM - evaluate if date field is null

Hello,

I'm trying to build a process that evaluates if a date field is null. Whether I try this with a business rule or a gateway, available conditions in SugarBPM do not include an option to evaluate if a date field is null. Does anyone have suggestions on how I can accomplish this? 

Trigger = user selects checkbox field 
 then check if date field is null
  if date is null
   then date = today()
   else do not update date
end

Happy to hear how others have gotten around this.

Kind regards,