How does PostgreSQL treat null values in unique composite constraints???

I have the following table with a unique constraint.

create table testt (
id int,
aa int,
bb int,
constraint pk_testt primary key (id),
constraint cons_testt unique (aa,bb));

Let's say I insert the following:

insert into testt values (1,2,null);
insert into testt values (2,2,null);

I thought this should fail because of the unique
constraint, but it doesn't.  How does PostgreSQL view
the null values in this constraint?  Will someone
explain why I am allowed to make this insertion?

TIA.

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