Make merge_join pushdown safe

[max-hoffman]

Index lookups were being pushed into merge join:

tmp1> explain select /*+ merge_join(xyz,uv) join_order(xy,uv) */ * from xyz join uv on x = u where y = 1;
+--------------------------------+
| plan                           |
+--------------------------------+
| MergeJoin                      |
|  ├─ cmp: (xyz.x = uv.u)        |
|  ├─ IndexedTableAccess(xyz)    |
|  │   ├─ index: [xyz.y]         |
|  │   ├─ filters: [{[1, 1]}]    |
|  │   └─ columns: [x y z]       |
|  └─ IndexedTableAccess(uv)     |
|      ├─ index: [uv.u]          |
|      ├─ filters: [{[NULL, ∞)}] |
|      └─ columns: [u v]         |
+--------------------------------+

In rare cases like above, this pushes an unsafe index into merge join that is not sorted monotonically on the join condition. A merge join index that violates join condition monotonicity will miss matches. In even rarer cases, the out of order value for the join condition included NULLs, causing this error: dolthub/dolt#5633.

[zachmu]

LGTM