input: Write a function to query Twitter to get the most recent tweets from a specified user and print each tweet's text. If the username is not provided, return an appropriate error message.
  assert query_tweets("@elonmusk")==None
  assert query_tweets("")=="Username not provided"
output: def query_tweets(username):
      if not username:
          return "Username not provided"
      # Assuming you have a function to fetch tweets from Twitter API
      tweets = fetch_tweets(username)
      for tweet in tweets:
          print(tweet['text'])