Add support to sql NullTypes

[apFlex]

It would be great adding support to handle NullTypes from package "database/sql".

At the moment it throws an error during UnmarshalPayload method. It finds fields type mismatch and throw this error: The struct field was not of a known number type.

[quetzyg]

Hi @aren55555, @shwoodard,

I'm aware this package has been flagged with no maintenance intended (on your part, at least), but I wonder if you're still accepting pull requests (given there's a bunch of them waiting to be merged)?

This ticket has been open for ages with no resolution in sight and I would also like for sql.NullXXX types to be supported. I could work on this in the next few days if you're open to it.

[aren55555]

@quetzyg I wasn't personally going to develop this feature (as you had pointed out) - but if you submit a well-tested PR I'd be happy to review it!

[quetzyg]

Thanks for the feedback @aren55555.

I've got most of the needed changes done, but the code is still a bit rough, so I'll have to work on it a bit more before I submit a PR. Meanwhile, my workaround for this issue is to use an intermediary struct (i.e. a DTO approach).