tmux: fix regression after f584de59

Fixes #7041

plugins/tmux/tmux.plugin.zsh

@@ -60,9 +60,7 @@ function _zsh_tmux_plugin_run() {
   [[ "$ZSH_TMUX_ITERM2" == "true" ]] && tmux_cmd+=(-CC)
 
   # Try to connect to an existing session.
-  if [[ "$ZSH_TMUX_AUTOCONNECT" == "true" ]]; then
-    $tmux_cmd attach
-  fi
+  [[ "$ZSH_TMUX_AUTOCONNECT" == "true" ]] && $tmux_cmd attach
 
   # If failed, just run tmux, fixing the TERM variable if requested.
   if [[ $? -ne 0 ]]; then