Edit: It is the same solution that @DanT described.
I think I have a simpler solution:
Create a new variable called nobody_at_the_door, which is the same as someone_at_the_door but required variable state is FALSE
Then you need to logicblocks:
Logicblock 1. welcoming_foyer_light_ON
Conditions: someone_at_the_door AND dark_outside
Actions for positive result: turn_light_on
Logicblock 2. welcoming_foyer_light_OFF
Conditions: nobody_at_the_door AND dark_outside
Actions for positive result: turn_light_off