If you are only adding the order temporarily with no customer id until a customer is defined, would it not be simpler to add the customer and order in a single transaction, thereby removing the need for the NULL foreign key entry and avoiding any constraints or triggers you've set up being violated?
Normally this situation arises ins web apps where the order is detailed before the customer defines who he/she is. And in those situations the order is kept in server state or in a cookie until all the necessary state for a complete order is supplied at which point the order is persisted to the database.
NULL foreign keys are ok for things like addresses, as mentioned above. But a NULL customer field doesn't make sense for an order and should be constrained.
