Fixs notifier when display_on_all_clients is true

line 254 no spaces '\#{client_tty}' are important
Remove null terminal from clients list

Starting tmux in control mode ```tmux -C``` results in (null) being
listed in client list ```tmux list-clients -F '#{client_tty}'```
Merge pull request #583 from mrloop/tmux_notifier_fix

Fixes tmux notifier when display_on_all_clients is true