MDEV-25236 fixup: instant_alter_debug,dynamic result

We only support the instantaneous removal of the NOT NULL attribute for
ROW_FORMAT=REDUNDANT tables.

mysql-test/suite/innodb/r/instant_alter_debug,dynamic.rdiff

+@@ -470,4 +470,4 @@
+ FROM information_schema.global_status
+ WHERE variable_name = 'innodb_instant_alter_column';
+ instants
+-33
++32