0..1:* relationship

Question

 Relationship(c => c.Department).IsRequired()
                .FromProperty(d => d.Courses).HasConstraint(
                (c, d) => c.DepartmentID == d.DepartmentID);

 Relationship(c => c.Department)
                .FromProperty(d => d.Courses).HasConstraint(
                (c, d) => c.DepartmentID == d.DepartmentID);

 Relationship(c => c.Department)
                .HasConstraint(
                (c, d) => c.DepartmentID == d.DepartmentID);

All have the same result : 1...* relationship

I want to have 0...* relationship,and read DepartmentID directly ,how can i set

DepartmentID =0 (NULL)

ParentId=0 (NULL)