Make empty default on_data_arrived implementation

src/connection_handler.hpp

@@ -39,8 +39,8 @@ public:
    * after the call to on_data_arrived.
    * @note May be called from an arbitrary network thread.
    */
-  virtual void on_data_arrived(std::string_view data,
+  virtual void on_data_arrived(std::string_view /*data*/,
-                               std::unique_ptr<Connection> &conn_ptr) = 0;
+                               std::unique_ptr<Connection> &) {};
 
   /**
    * Successfully wrote data on the connection.