rpc_client: Prevent Thallium automatic conversions when not needed

    address() const;

    template <typename... Args>

    auto

    inline std::optional<thallium::packed_data<>>

    call(const std::string& rpc_name, Args&&... args) const {

        // deduce the return type of the expression in the try-block below so

        // that we know the type to return within std::optional

        using rpc_function_type =

                decltype(m_engine->define(std::declval<decltype(rpc_name)>()));

        using rpc_return_type = decltype(std::declval<rpc_function_type>().on(

                m_endpoint)(std::forward<Args>(args)...));

        using return_type = std::optional<rpc_return_type>;

        try {

            const auto& rpc = m_engine->define(rpc_name);

            const auto& rv = rpc.on(m_endpoint)(std::forward<Args>(args)...);

            return return_type{rv};

            const auto rpc = m_engine->define(rpc_name);

            return std::make_optional(

                    rpc.on(m_endpoint)(std::forward<Args>(args)...));

        } catch(const std::exception& ex) {

            LOGGER_ERROR("endpoint::call() failed: {}", ex.what());

            return return_type{};

            return std::nullopt;

        }

    }