Bug in get_partition_for_tuple

Just observed a crash due to thinko in the logic that handles NULL
partition key. Absence of null-accepting partition in this case should
have caused an error, instead the current code proceeds with comparison
resulting in crash.

create table p (a int, b char) partition by list (b);
create table p1 partition of p for values in ('a');
insert into p values (1); -- crashes

Attached patch fixes that and adds a test.

Thanks,
Amit