Set to Minor because only 10.2 and 10.3 are affected.

CREATE TABLE t (b TEXT NOT NULL);

ALTER TABLE t ALTER b SET DEFAULT NULL;

SHOW CREATE TABLE t;

The ALTER is obviously wrong, as it attempts to set the default value to NULL while the column is not null. But it works, and the resulting table is this:

CREATE TABLE `t` (

  `b` text NOT NULL DEFAULT NULL

) ENGINE=MyISAM DEFAULT CHARSET=latin1

Reproducible on 10.2-10.3.
Not reproducible on 10.4-10.6.

An attempt to execute this CREATE statement on any version, including the affected ones, expectedly ends with an error:

query 'CREATE TABLE `t` (

`b` text NOT NULL DEFAULT NULL

) ENGINE=MyISAM DEFAULT CHARSET=latin1

' failed: 1067: Invalid default value for 'b'