Why is this kind of foreign keys possible

create table prova_a (a number, b number);
alter table prova_a add primary key (a,b);
create table prova_b (a number, b number);
alter table prova_b add foreign key (a,b) references prova_a(a,b) ;
insert into prova_a  values (1,2);

insert into prova_b  values (123,null);
insert into prova_b  values (null,123);

Msg 8111, Level 16, State 1, Line 2
Cannot define PRIMARY KEY constraint on nullable column in table 'prova_a'.
Msg 1750, Level 16, State 0, Line 2
Could not create constraint. See previous errors.