Hi there! First of all, thanks for this library -- it's been fantastic to use.

I'm trying to emit some well-formed SARIF in zizmor and I'm running into some weirdness with how ResultLevel and ResultKind are serialized. More precisely, they seem to always be serialized as null (i.e. serde_json::value::Value::Null), regardless of their actual value.

Here's a minimal failing reproducer:

#[cfg(test)]
mod tests {
    use serde_sarif::sarif::ResultKind;

    #[test]
    fn test_resultkind_serializes() {
        assert_eq!(
            serde_json::to_string(&ResultKind::Fail).unwrap(),
            "\"fail\""
        );
    }
}

The expected behavior is that this serializes to the JSON string "fail", but instead it serializes as null:

assertion `left == right` failed
  left: "null"
 right: "\"fail\""
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

I took a quick look at the code, and it doesn't look wrong, but I'm not super familiar with strum (which seems to be controlling the serialization equivalents for each enum variant).

Let me know if there's any other information I can provide! I'll also keep debugging this on my end.