PG11 Hash partitioning and null values in the partition key

Hi,

given this setup:

create table part2 ( a int, list varchar(10) ) partition by hash (a);
create table part2_1 partition of part2 FOR VALUES WITH (MODULUS 3, REMAINDER 0);
create table part2_2 partition of part2 FOR VALUES WITH (MODULUS 3, REMAINDER 1);
create table part2_3 partition of part2 FOR VALUES WITH (MODULUS 3, REMAINDER 2);

insert into part2 (a,list) values (1,'aaa');
insert into part2 (a,list) values (2,'bbb');
insert into part2 (a,list) values (3,'ccc');

... it is possible to insert rows like this which will always go to the first partition:

insert into part2 (a,list) values (null,'ddd');
insert into part2 (a,list) values (null,'eee');
select * from part2_1;
a | list
---+------
2 | bbb
| ddd
| eee
(3 rows)

I suppose this is intended but I could not find anything in the documentation about that. Can someone please clarify the logic behind that?

Thanks in advance
Daniel