fix: NOT IN with NULL subquery returns wrong results under SortMergeJoin

[nathanb9]

Problem

NOT IN (subquery) is a null-aware anti join: when the subquery yields a NULL the predicate is never TRUE, so the query must return zero rows. With prefer_hash_join = false and multiple partitions, the planner routed the null-aware anti join to SortMergeJoinExec, which is not null-aware, so it returned wrong results. HashJoin (the default) was already correct.

Proof

set datafusion.optimizer.prefer_hash_join = false;
create table t1(x int) as values (1);
create table t2(y int) as values (NULL);
select x from t1 where x not in (select y from t2);

Expected 0 rows (the subquery contains a NULL). Before this change it returned 1. With prefer_hash_join = true it correctly returned 0 rows. EXPLAIN showed the wrong config selecting SortMergeJoinExec: join_type=LeftAnti.

Solution

The planner already requires null-aware joins to use the CollectLeft HashJoin, and the HashJoin branch guards on !null_aware. The SortMergeJoin branch was missing the same guard, so this adds && !*null_aware to it. Null-aware anti joins now fall through to the CollectLeft HashJoin regardless of prefer_hash_join. SortMergeJoinExec has no null_aware parameter and cannot honor these semantics.

Added a regression test in subquery.slt (under prefer_hash_join = false) covering both a null-containing subquery (zero rows) and a null-free subquery (normal anti join). All 61 SortMergeJoin unit tests pass.