Add default CLI response impl

[hu55a1n1]

Summary

Seems like most of our CLI commands don't have anything specific to output, so we should provide a Response::default() impl that just serializes to an empty string.

Proposal

• Impl Default for Response. e.g.

#[derive(Clone, Debug, Serialize)]
#[serde(untagged)]                         // <--------------------
pub enum Response {
    String(String),
    // ...
}

impl Default for Response {
    fn default() -> Self {
        Self::String("".to_string())
    }
}

• Or make Response optional in the Handler trait, e.g.

    fn handle(self) -> Result<Option<Self::Response>, Self::Error>;