input: Write a function to parse a JSON data structure and return the values of the 'name', 'age', and 'occupation' keys. If the input is not a valid JSON string, return None.
  assert parse_json('{"name": "John", "age": 18, "occupation": "Software Engineer"}')==("John", 18, "Software Engineer")
  assert parse_json('{"name": "Jane", "age": 25, "occupation": "Doctor"}')==("Jane", 25, "Doctor")
  assert parse_json('{"name": "Bob", "age": 45, "occupation": "Teacher"}')==("Bob", 45, "Teacher")
  assert parse_json('{"name": "Alice", "age": 30, "occupation": "Lawyer"}')==("Alice", 30, "Lawyer")
  assert parse_json('{"name": "Tom", "age": 22, "occupation": "Engineer"}')==("Tom", 22, "Engineer")
output: try:
      data = json.loads(json_str)
      return (data.get('name'), data.get('age'), data.get('occupation'))
  except json.JSONDecodeError:
      return None