Do not allow sending empty messages (BB-2529)

The bitbucket messaging system allows for sending empty messages via the web interface, which in my eyes can be considered a bug.

I've hit the "send message" button two times, because I thought this would open the message form. However, I did not see the actual form already existed because of a combination of thin borders/bad screen/bad viewing angle. So I sent two emtpy messages without purpose. It would be good if bitbucket would at least ask for a confirmation before sending empty messages.

Jan-Philip