Overflow partition and NULL values

drop table Test_Partitioning;
drop partition scheme Test_Partition_Scheme;
drop partition function Test_Partition_Function;

create partition function Test_Partition_Function (datetime)
as range left for values (null);

create partition scheme Test_Partition_Scheme
as partition Test_Partition_Function
all to ([PRIMARY]);

create table Test_Partitioning (
    id int not null,
    stamp datetime null
) ON Test_Partition_Scheme (stamp);

insert into Test_Partitioning values (1, '2001-01-01');
insert into Test_Partitioning values (2, null);

SELECT
    $partition.Test_Partition_Function(t.stamp) AS [Partition Number],
    t.*
FROM
    Test_Partitioning t

/*
Partition Number    id  stamp
1           2   NULL
2           1   2001-01-01 00:00:00.000
*/