input: Write a function to visualize the Twitter data by creating bar plots for the number of tweets per author, the number of likes per tweet, and the number of retweets per tweet. The Twitter data is a stream of tweets with data fields like tweet author, date of tweet, text, number of likes, and number of retweets.
  
  assert visualize_twitter_data([])==None
  assert visualize_twitter_data([{"author": "Alice", "likes": 10, "retweets": 5}, {"author": "Bob", "likes": 15, "retweets": 7}, {"author": "Alice", "likes": 8, "retweets": 3}])==None
  assert visualize_twitter_data([{"author": "Charlie", "likes": 20, "retweets": 12}, {"author": "Charlie", "likes": 22, "retweets": 15}, {"author": "David", "likes": 17, "retweets": 9}])==None
output: def visualize_twitter_data(tweets):
      if not tweets:
          return None
      
      authors = [tweet['author'] for tweet in tweets]
      likes = [tweet['likes'] for tweet in tweets]
      retweets = [tweet['retweets'] for tweet in tweets]
      
      plt.figure(figsize=(15, 5))
      
      plt.subplot(1, 3, 1)
      plt.bar(authors, likes)
      plt.title('Likes per Tweet')
      plt.xlabel('Author')
      plt.ylabel('Likes')
      
      plt.subplot(1, 3, 2)
      plt.bar(authors, retweets)
      plt.title('Retweets per Tweet')
      plt.xlabel('Author')
      plt.ylabel('Retweets')
      
      plt.subplot(1, 3, 3)
      author_counts = {author: authors.count(author) for author in set(authors)}
      plt.bar(author_counts.keys(), author_counts.values())
      plt.title('Number of Tweets per Author')
      plt.xlabel('Author')
      plt.ylabel('Number of Tweets')
      
      plt.tight_layout()
      plt.show()
      
      return None