Work around for issue in vscode reporting a failure in request

vscode would report "A request has failed" when it got "Content modified"
message and this would cause a pop-up to appear. This works around the issue by
returning an "empty" response that vscode can ignore.

crates/gen_lsp_server/src/msg.rs

@@ -120,6 +120,11 @@ impl RawResponse {
         let error = RawResponseError { code, message, data: None };
         RawResponse { id, result: None, error: Some(error) }
     }
+    /// Returns an "empty" RawResponse
+    /// Empty response still has a value `()` in its `result` field.
+    pub fn empty(id: u64) -> RawResponse {
+        RawResponse { id, result: Some(to_value(&()).unwrap()), error: None }
+    }
 }
 
 impl RawNotification {