fix: use double brackets to avoid 'too many arguments' error

An error occured when the trigger was assigned to '('

shell/completion.zsh

@@ -323,7 +323,7 @@ fzf-completion() {
 
   # Explicitly allow for empty trigger.
   trigger=${FZF_COMPLETION_TRIGGER-'**'}
-  [ -z "$trigger" -a ${LBUFFER[-1]} = ' ' ] && tokens+=("")
+  [[ -z $trigger && ${LBUFFER[-1]} == ' ' ]] && tokens+=("")
 
   # When the trigger starts with ';', it becomes a separate token
   if [[ ${LBUFFER} = *"${tokens[-2]-}${tokens[-1]}" ]]; then