better mysql JSON type handling

[frankli0324]

What do you want to change?

https://github.com/sqlc-dev/sqlc/pull/652 introduced mapping JSON type to json.RawMessage, however json.RawMessage couldn't handle nil values (since it's not a sql.Scanner) it should be like how pg handles NULLable types https://github.com/sqlc-dev/sqlc/commit/2143e467fd50bc381390c6795b7dcffb74a4b7a3

The question is which type to use

What database engines need to be changed?

MySQL

What programming language backends need to be changed?

Go