Function invoke_empty 

pub async fn invoke_empty<U, O>(
    upstream: &U,
    target: &U::Target,
    request: WireRequest<()>,
    limits: CodecLimits,
) -> Result<JsonInvocation<O>, TransformError>
where
    U: Upstream,
    O: DeserializeOwned + DeclaredFields,

Send one request with an empty HTTP body and decode its JSON success body. This is suitable for models/files GET operations; () is not encoded as JSON null. Non-2xx bodies are returned untouched, as with invoke_json.