input: Write a function to store and retrieve user data from a JSON file. The function should first store the user data and then be able to retrieve it from the file. If the JSON file does not exist, it should return an empty dictionary.
  
  def store_user_data(data):
      with open('user_data.json', 'w') as f:
          json.dump(data, f)
  
  def get_user_data():
      try:
          with open('user_data.json') as f:
              return json.load(f)
      except FileNotFoundError:
          return {}
          
  assert store_user_data({"Name": "John", "Age": 25, "City": "New York"})==None
  assert get_user_data()=={"Name": "John", "Age": 25, "City": "New York"}
  assert store_user_data({"Name": "Alice", "Age": 30, "City": "London"})==None
  assert get_user_data()=={"Name": "Alice", "Age": 30, "City": "London"}
output: def store_user_data(data):
      with open('user_data.json', 'w') as f:
          json.dump(data, f)
  
  def get_user_data():
      try:
          with open('user_data.json') as f:
              return json.load(f)
      except FileNotFoundError:
          return {}