Default value set null when modify a column from nullable to not null

There's a relational issue DBZ-771, it solved the bug when execute statement alter table change about default value,
but, statement alter table modify also cause the same issue.

Just extend DBZ-771's test case,
io.debezium.connector.mysql.MysqlDefaultValueIT.columnTypeChangeResetsDefaultValue(),
continue execute these two SQLs

alter table DBZ_771_CUSTOMERS modify customer_type int null;
alter table DBZ_771_CUSTOMERS modify customer_type int not null;

will cause Exception:

......
Caused by: org.apache.kafka.connect.errors.DataException: Invalid value: null used for required field: "null", schema type: INT32
	at org.apache.kafka.connect.data.ConnectSchema.validateValue(ConnectSchema.java:220)
	at org.apache.kafka.connect.data.ConnectSchema.validateValue(ConnectSchema.java:213)
	at org.apache.kafka.connect.data.SchemaBuilder.defaultValue(SchemaBuilder.java:129)
	... 19 more

Caz after apply alter table DBZ_771_CUSTOMERS modify customer_type int not null; this column definition is customer_type int not null default value null;, it should reset default value just same as parsing alter table change