The ASQN attribute has an intrinsic method .AsValue. If you want to control what value will be returned from an expression if the value of a field is SQL Null, use the AsValue() intrinsic method for the field. The AsValue parameter will be returned instead of SQL Null when the field's value is SQL Null.
Also See
ASQN (Allow SQL Nulls) attribute
Assignment, Conditions, and Expressions with Fields allowing SQL Null
