It's much easier to do in dynamically typed languages or by using maps - agreed. Unfortunately we have other design decisions that force us into using structs for deserialization on this endpoint (part of our validation strategy.)

The Go answer is a struct that contains an IsDefined boolean (i.e. your first option), and a pointer-to-value (i.e. your second option.)

This is fine if you need it, but having to write this same logic over and over again for every type gets old... especially if your validators are tied to your types (i.e. a type per field.)