codegen: fix required ID deserialization

[ajwerner]

Fixes #522

[tomhoule]

Good catch. Thanks for the fix!

[danieleades]

this was merged with failing CI jobs, will need to be fixed in a new PR to get the builds passing again

[Sytten]

This broke a bunch of stuff and needs to be reverted or improved:

`?` operator cannot convert from `std::string::String` to `std::vec::Vec<std::string::String>`
expected struct `std::vec::Vec<std::string::String>`
   found struct `std::string::String`

= note: `?` operator cannot convert from `std::string::String` to `std::option::Option<std::vec::Vec<std::string::String>>`
   = note: expected enum `std::option::Option<std::vec::Vec<std::string::String>>`
            found struct `std::string::String`