Add example showing how to handle empty vs missing query params (#443)

src/extract/query.rs

@@ -39,6 +39,11 @@ use std::ops::Deref;
 ///
 /// If the query string cannot be parsed it will reject the request with a `400
 /// Bad Request` response.
+///
+/// For handling values being empty vs missing see the (query-params-with-empty-strings)[example]
+/// example.
+///
+/// [example]: https://github.com/tokio-rs/axum/blob/main/examples/query-params-with-empty-strings/src/main.rs
 #[derive(Debug, Clone, Copy, Default)]
 pub struct Query<T>(pub T);