Relationships test always passes when there are nulls in the foreign key

[cmcarthur]

Issue

Issue description

When running this SQL on Redshift:

with a as (
  select null as id
    union all
  select 2 as id
)
select 1 as id
where id is not null
and id not in (
  select id
  from a
);

you would expect this to return 1 row. But it actually returns 0 rows. We need to either say that this is the correct behavior, or we need to fix it. The fix is just to add a WHERE ... IS NOT NULL in the subquery.