Find the average number of followers for the users who do not have any tweet.
return users ;return tweets of #1 ;return number of  #2 for each  #1 ;return #1 where #3 is zero ;return followers of #4 ;return number of  #5 for each  #4 ;return average of #6
['select', 'project', 'group', 'comparative', 'project', 'group', 'aggregate']