`EliminateCrossJoin` drops `NullEqualsNull` from nested inner joins

[neilconway]

Describe the bug

EliminateCrossJoin flattens every inner join beneath an inner-join root, merges all their equijoin keys, and rebuilds the joins using only the root join's null_equality. If a nested inner join carries NullEqualsNull, the rebuilt join silently downgrades it to NullEqualsNothing, and NULL keys stop matching.

To Reproduce

CREATE TABLE t1(a INT, b INT) AS VALUES (NULL, 1), (5, 2);
CREATE TABLE t2(a INT) AS VALUES (NULL), (5);
CREATE TABLE t3(b INT) AS VALUES (1), (2);

-- Correct: 2 rows, NULL matches NULL
SELECT t1.a, t1.b, t2.a
FROM t1 JOIN t2 ON t1.a IS NOT DISTINCT FROM t2.a
ORDER BY t1.b;

-- Wrong: 1 row, the NULL-key row is gone
SELECT t1.a, t1.b, t2.a, t3.b
FROM t1 JOIN t2 ON t1.a IS NOT DISTINCT FROM t2.a
        JOIN t3 ON t1.b = t3.b
ORDER BY t1.b;

Output:

+------+---+------+
| a    | b | a    |
+------+---+------+
| NULL | 1 | NULL |
| 5    | 2 | 5    |
+------+---+------+
2 row(s) fetched.
Elapsed 0.022 seconds.

+---+---+---+---+
| a | b | a | b |
+---+---+---+---+
| 5 | 2 | 5 | 2 |
+---+---+---+---+
1 row(s) fetched.
Elapsed 0.002 seconds.

Expected behavior

Both queries should return the NULL-key row.

Additional context

No response