Middleware layer that removes params with empty values.

In nginx configuration, when rewriting URLs (e.g. to pass to the upstream server), query params can be interpolated using $arg_

Example:

proxy_pass http://upstream/some/route?format=$arg_param;

Axum makes it easy to write handlers that automatically deserialize the query parameters:

#[derive(Deserialize)]
enum Format {
    Json,
    Html,
    Text
}

#[derive(Deserialize)]
struct RouteArgs {
    format: Option<Format>
}

Annoyingly, the above example will fail. Since format= is always unconditionally passed to the handler via the nginx configuratio, the default deserialization will try to deserialize Format from "".