How to remove the send without subject message?

I use the latest Thunderbird email program. I send a lot of emails to text addresses. So I do not use a subject line. Each time I send a message it comes up with a warning "send message without subject?" Is there a way to override this and send without getting this message every single time? In other words get this pop up to not show up again? Something easy for someone who is not a computer guru? It would be nice like most popups, to have a do not show this message again button. Or did I miss this in settings somewhere??