Create a default tmux session only when none defined in tmux.conf

Signed-off-by: Sorin Ionescu <sorin.ionescu@gmail.com>

modules/tmux/init.zsh

@@ -21,12 +21,12 @@ if [[ -z "$TMUX" && -z "$EMACS" && -z "$VIM" ]] && ( \
   ( [[ -n "$SSH_TTY" ]] && zstyle -t ':prezto:module:tmux:auto-start' remote ) ||
   ( [[ -z "$SSH_TTY" ]] && zstyle -t ':prezto:module:tmux:auto-start' local ) \
 ); then
-  tmux_session='prezto'
+  tmux start-server
 
-  # Create a first 'prezto' session if tmux is starting.
+  # Create a 'prezto' session if no session has been defined in tmux.conf.
   if ! tmux has-session 2> /dev/null; then
+    tmux_session='prezto'
     tmux \
-      start-server \; \
       new-session -d -s "$tmux_session" \; \
       set-option -t "$tmux_session" destroy-unattached off &> /dev/null
   fi