verilog module to NULL

I created a complex module.
Not all output using all the time.
How can I redirect the unused outputs too "null".

sample:

 uart_rx rx1 (.reset(uart_rst),
  .rxclk(baud_clk),
  .rx_read(rs),
  .rx_data(data),
  .rx_in(rx),
  .rx_empty( to null),
  .rx_busy(to null)