Add support for passing NULL parameters.

[paurkedal]

I'm working on paurkedal/ocaml-caqti#2, I need a way to pass NULL parameters, since I support option types. Mariadb has two ways to do it, with the is_null or by declaring the field to be null by type. The latter seemed natural as it only required adding `Null to the variant used for parameter passing.

[andrenth]

Looks good to me.

[paurkedal]

Thanks!