codegen: breaking change for `ID!` types

[ajwerner]

#476 is a welcome enough change, but breaks code which use ID! types in their schemas. The function used for deserialization returns an Option<String> even if the field is String:

     |
2140 |         #[derive(Deserialize, Serialize, Debug)]
     |                  ^^^^^^^^^^^ expected `String`, found `Option<String>`

cc @willfindlay / @tomhoule.