fix: MVCC cursor ignored left join null flag

[jussisaurio]

turso> pragma journal_mode='experimental_mvcc';
turso> CREATE TABLE t1(x INTEGER PRIMARY KEY);
CREATE TABLE t2(x INTEGER PRIMARY KEY, v TEXT);
INSERT INTO t1 VALUES (1);
INSERT INTO t2 VALUES (1, 'hello');
SELECT t1.x, t2.x FROM t1 LEFT JOIN t2 ON t1.x = t2.x AND t2.v = 'nonexistent';
┌───┬───┐
│ x │ x │
├───┼───┤
│ 1 │ 1 │
└───┴───┘

turso> CREATE TABLE t1(x INTEGER PRIMARY KEY);
CREATE TABLE t2(x INTEGER PRIMARY KEY, v TEXT);
INSERT INTO t1 VALUES (1);
INSERT INTO t2 VALUES (1, 'hello');
SELECT t1.x, t2.x FROM t1 LEFT JOIN t2 ON t1.x = t2.x AND t2.v = 'nonexistent';
┌───┬───┐
│ x │ x │
├───┼───┤
│ 1 │   │
└───┴───┘

fix: put a shared null flag check in op_row_id so both mvcc/btree cursors check it.