input: Write a function to parse a JSON data structure. If the given data is empty, return None for all values.
  
  def parse_json(data):
      if not data:
          return None, None, None
      data = json.loads(data)
      return data['name'], data['age'], data['occupation']
  
  assert parse_json('{"name": "John", "age": 18, "occupation": "Software Engineer"}')==('John', 18, 'Software Engineer')
  assert parse_json('{"name": "Alice", "age": 25, "occupation": "Data Scientist"}')==('Alice', 25, 'Data Scientist')
  assert parse_json('{}')==(None, None, None)
  assert parse_json('')==(None, None, None)
output: import json
  
  def parse_json(data):
      if not data:
          return None, None, None
      data = json.loads(data)
      return data.get('name'), data.get('age'), data.get('occupation')