@Digico answer may work. I'm not sure if a digital item might also cause that setting to be included, or how you would want to handle split fulfillment scenarios.
The fulfillment location is assigned after an order is created to a "Fulfillment Order". You could check:
is at least one of order / fulfillmentOrders / deliveryMethod / methodType == "PICK_UP"
If you do that, you'll either want to use Order ready to fulfill as the trigger (it fires after the fulfillment order is assigned), or add a wait step of about 5 min after the trigger to make sure the system has time to assign the right location.