message: fix nightly rustdoc-extern tests
diff: @@ -52,7 +52,8 @@ impl<'de> serde::de::Deserialize<'de> for RustdocExternMode {
  }
  }
  -#[derive(serde::Deserialize, Debug)]
  +#[derive(serde::Deserialize, Debug, Default)]
  +#[serde(default)]
  pub struct RustdocExternMap {
  registries: HashMap<String, String>,
  std: Option<RustdocExternMode>,
  