input: Write a function to store and retrieve user data in a JSON file. The function should be able to store the given user_data dictionary to a file named 'user_data.json' and retrieve the data back from the file. If the file does not exist, the function should return an empty dictionary.
  assert store_user_data(user_data)
  assert get_user_data() == {
      "Name": "John", 
      "Age": 25, 
      "City": "New York"
  }
  assert store_user_data({"Name": "Jane", "Age": 30, "City": "Los Angeles"})
  assert get_user_data() == {
      "Name": "Jane", 
      "Age": 30, 
      "City": "Los Angeles"
  }
  assert get_user_data() == {} # if the file does not exist
output: def store_user_data(user_data):
      with open('user_data.json', 'w') as file:
          json.dump(user_data, file)
  
  def get_user_data():
      try:
          with open('user_data.json', 'r') as file:
              return json.load(file)
      except FileNotFoundError:
          return {}