config: Default WireGuard::extra_params to empty Vec

Otherwise, omitting `extra_params` in the configuration file will result in a `WireGuard` configuration object of `None`, even though not specifying `extra_params` is sane.

src/config.rs

@@ -55,6 +55,8 @@ pub struct RosenpassPeer {
 pub struct WireGuard {
     pub device: String,
     pub peer: String,
+
+    #[serde(default)]
     pub extra_params: Vec<String>,
 }