Why, in the solution, there's no NOT NULL included in each column? Are we supposed to not have information of customers included?

yeah there should be not null clause for customer_id since it will act as primary key in relationship schema

I would recommend that the solution file be corrected. On the one hand, we are learning that the primary key MUST be given as NOT NULL values, while on the other hand, you are claiming that we are not working with real data! It's a little odd. Whether it is or not, we need a proper schema in place; aren't we expected to do it here?