input: Write a python function to get user information from a database based on the username.
  assert get_user_info('john_doe') == {'username': 'john_doe', 'email': 'john@example.com', 'age': 30}
  assert get_user_info('jane_doe') == {'username': 'jane_doe', 'email': 'jane@example.com', 'age': 25}
  assert get_user_info('unknown_user') == None
output: def get_user_info(username):
      database = {
          'john_doe': {'username': 'john_doe', 'email': 'john@example.com', 'age': 30},
          'jane_doe': {'username': 'jane_doe', 'email': 'jane@example.com', 'age': 25}
      }
      return database.get(username)