Null AK?

An Alternate Key (AK) is a unique identifier for an entity instance that has not been selected as the primary key. In the following Order entity for example, Order_Identifier is the primary key and Order_Number is the alternate key:

Order_Number is the alternate key yet it is defined on this model as null (meaning it could be left empty).

Can an alternate key ever be null? If yes, under what situations can it be null? If no, why not?