Unit structs encode differently between sval_json and serde_json

[KodrAus]

#[derive(sval::Value, serde::Serialize)]
pub struct MyStruct;

fn main() {
    println!("{}", sval_json::stream_to_string(Some(MyStruct)).unwrap());
    println!("{}", serde_json::to_string(&Some(MyStruct)).unwrap());
}

prints:

"MyStruct"
null

In sval_json, we print the name of the unit struct because we treat them the same as unit variants. In serde_json, unit structs are encoded as null.

sval_json attempts to be serde_json compatible in encoding, so this is worth calling out, but can't be changed at this point.