DEFAULT NULL for NOT NULL columns is not always detected

MariaDB [test]> create table t1 (a int not null default null);

ERROR 1067 (42000): Invalid default value for 'a'

MariaDB [test]> create table t1 (a int default null primary key);

ERROR 1067 (42000): Invalid default value for 'a'

MariaDB [test]> create table t1 (a int default null, primary key (a));

Query OK, 0 rows affected (0.014 sec)

MariaDB [test]> show create table t1\G

*************************** 1. row ***************************

       Table: t1

Create Table: CREATE TABLE `t1` (

  `a` int(11) NOT NULL,

  PRIMARY KEY (`a`)

) ENGINE=MyISAM DEFAULT CHARSET=latin1

1 row in set (0.001 sec)