input: Write a function to parse a JSON data structure and extract the values of the 'name', 'age', and 'occupation' keys. If any of these keys do not exist, return a tuple of three empty strings. If the input is not a valid JSON, return 'Invalid JSON'."\nassert parse_json('{\"name\":\"John\", \"age\":18, \"occupation\":\"Software Engineer\"}') == ('John', 18, 'Software Engineer')
  assert parse_json('{\"name\":\"John\", \"age\":18}') == ('', '', '')
  assert parse_json('This is not a JSON') == 'Invalid JSON'
  assert parse_json('') == 'Invalid JSON'
output: def parse_json(json_str):
      try:
          data = json.loads(json_str)
          name = data.get('name', '')
          age = data.get('age', '')
          occupation = data.get('occupation', '')
          return (name, age, occupation)
      except json.JSONDecodeError:
          return 'Invalid JSON'