Added possibility to return empty JSON object instead of NULL

[killoctal]

The problem is that having an empty json object {} is complicated to obtain because the serialization always outputs null.

With this method it is easy to have the desired output. Of course, it can be used as response or as a value.

#include "crow.h"

int main()
{
    MyProgram program;
    crow::SimpleApp app;

    // This route will return "{}" instead of "null"
    CROW_ROUTE(app, "/notify-alive")([](){
        program.notify_alive();
        return crow::json::wvalue::empty_object();
    });

    app.port(18080).run();
}