better mysql JSON type handling

[frankli0324]

What do you want to change?

#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 2143e46

The question is which type to use

What database engines need to be changed?

MySQL

What programming language backends need to be changed?

Go