input: Write a function to visualize the Twitter data by plotting summaries of tweet authors, number of likes, and number of retweets using matplotlib. If the input data is empty then return without plotting anything.
  assert visualize_twitter_data([{'author': 'Alice', 'date': '2023-01-01', 'text': 'Hello', 'likes': 10, 'retweets': 5}, {'author': 'Bob', 'date': '2023-01-02', 'text': 'Hi', 'likes': 20, 'retweets': 10}]) is None
  assert visualize_twitter_data([]) is None
output: def visualize_twitter_data(data):
      if not data:
          return
      
      authors = [tweet['author'] for tweet in data]
      likes = [tweet['likes'] for tweet in data]
      retweets = [tweet['retweets'] for tweet in data]
      
      fig, ax = plt.subplots(2, 1, figsize=(10, 8))
      
      ax[0].bar(authors, likes, color='blue')
      ax[0].set_title('Number of Likes by Author')
      ax[0].set_ylabel('Likes')
      
      ax[1].bar(authors, retweets, color='green')
      ax[1].set_title('Number of Retweets by Author')
      ax[1].set_xlabel('Author')
      ax[1].set_ylabel('Retweets')
      
      plt.tight_layout()
      plt.show()