The following tests shows we get different results depending on the setting of
'not_null_range_scan':

CREATE TABLE t1 (a INT, b INT) ENGINE=MyISAM;
INSERT INTO t1 VALUES (10,1),(null,2);
CREATE TABLE t2 (pk INT PRIMARY KEY) ENGINE=MyISAM;
INSERT INTO t2 VALUES (1),(2);
SET optimizer_switch= 'not_null_range_scan=on';
SELECT t1.b FROM t1 LEFT JOIN t2 ON t1.a = t2.pk WHERE t1.a IS NULL ORDER BY t1.b;
SET optimizer_switch= 'not_null_range_scan=off';
SELECT t1.b FROM t1 LEFT JOIN t2 ON t1.a = t2.pk WHERE t1.a IS NULL ORDER BY t1.b;

The first test returns 0 rows and the second test 1 row.