fix: set null_equals_null to false when convert_cross_join_to_inner_join

[jonahgao]

Which issue does this PR close?

Closes #11704.

Rationale for this change

Given an example query:

CREATE TABLE t1(a INT) AS VALUES(NULL);
CREATE TABLE t2(a INT) AS VALUES(NULL);
SELECT * FROM
  (SELECT * FROM t1 CROSS JOIN t2)
WHERE t1.a == t2.a
  AND t1.a + t2.a IS NULL;

The push_down_filter rule will convert the cross join into an inner join.

| initial_logical_plan                | Projection: t1.a, t2.a                                       |
|                                     |   Filter: t1.a = t2.a AND t1.a + t2.a IS NULL                |
|                                     |     Projection: t1.a, t2.a                                   |
|                                     |       CrossJoin:                                             |
|                                     |         TableScan: t1                                        |
|                                     |         TableScan: t2                                        |
| logical_plan after push_down_filter | Projection: t1.a, t2.a                                       |
|                                     |   Projection: t1.a, t2.a                                     |
|                                     |     Inner Join:  Filter: t2.a = t1.a AND t1.a + t2.a IS NULL |
|                                     |       TableScan: t1                                          |
|                                     |       TableScan: t2                                          |
| extract_equijoin_predicate          | Inner Join: t1.a = t2.a Filter: t1.a + t2.a IS NULL          |
|                                     |   TableScan: t1 projection=[a]                               |
|                                     |   TableScan: t2 projection=[a]                               |

null_equals_null is only valid for equijoins, and when applying the corresponding equality predicate to the cross join, null does not equal null. So we should set null_equals_null to false in order to convert the cross join to an equivalent equijoin.

What changes are included in this PR?

Are these changes tested?

Yes

Are there any user-facing changes?

No

[jonahgao]

The result of this query on the main branch is incorrect.

[alamb]

Thank you @jonahgao (and again @2010YOUY01 for the wonderful SQLancer testing)