Admit empty message On Free keyword input when the user don't send the correct response

I have a bot that on some moment I ask the user to send me an image. If the user writes eg “Ok!”, the bot should respond with something, that’s not the best for me. I need simply don’t sent anything when the user send to me an incorrect response, so I can put the retry times more than 0. So the chat will be more natural to the user.

Thanks!