Support "emit_pointers_for_null_types" for MySQL

[arieroos]

What do you want to change?

Hi

Thanks for this amazing library.

Will you please consider adding MySQL support for emit_pointers_for_null_types?

What database engines need to be changed?

MySQL

What programming language backends need to be changed?

Go

[arieroos]

I see there is already a PR for this: #3879