List current sessions if no session name was provided

First of, many thanks for the tmx script. I'm in the process of migrating from screen to tmux and tmx lets tmux behave just the way I want it to.

I found typing out "tmux list-sessions" too long, so I added it when no sessionname was given.
I think that when you always use predefined sessionnames this is not a big problem. I still think this is a nice addition.