input: Write a function to parse a JSON data structure and return the values associated with the keys 'name', 'age', and 'occupation'. If the input data is empty, return None for each key.
  assert parse_json('{"name": "John", "age": 18, "occupation": "Software Engineer"}')==('John', 18, 'Software Engineer')
  assert parse_json('{}')==(None, None, None)
  assert parse_json('{"name": "Alice"}')==('Alice', None, None)
  assert parse_json('{"age": 22, "occupation": "Student"}')==(None, 22, 'Student')
output: def parse_json(json_str):
      data = json.loads(json_str)
      name = data.get('name')
      age = data.get('age')
      occupation = data.get('occupation')
      return (name, age, occupation)