slave_type_conversions does not check for NOT NULL or DEFAULT differences

Consider the following replication situation:

• binlog_format=ROW
• slave_type_conversions is empty, so tables should be defined the same on master and slave for replication to work
• a column on the master is created with "NULL", but on the slave it has same type and size, but "NOT NULL DEFAULT ..." instead

E.g.:

• on master: CREATE TABLE t1(id int not null primary key, msg varchar(100) NULL);
• on slave: ALTER TABLE t1 MODIFY COLUMN msg varchar(100) NOT NULL DEFAULT 'abc';
• on master: INSERT INTO TABLE t1 VALUES(1, NULL); SELECT * FROM t1;
• -> result id:1; msg: NULL
• on slave: SELECT * FROM t1;
• -> result id: 1; msg: 'abc'

So now we have a data inconsistency between master and slave even though slave_type_conversions='' should have prevented this.

Double check to see that slave type conversion checks are indeed in effect:

• on master CREATE TABLE t2(id int not null primary key, msg varchar(100));
• on slave ALTER TABLE t2 MODIFY COLUMN msg varchar(200);
• on master: INSERT INTO t2 VALUES(1, NULL);
• on slave -> Last_SQL_Error: Column 1 of table 'test.t2' cannot be converted from type 'varchar(400 octets)' to type 'varchar(800 octets) character set utf8mb4'