input: Write a python function to move the empty tile up in the 3x3 puzzle.
  assert move_up([1, 0, 2, 3, 4, 5, 6, 7, 8]) == [1, 4, 2, 3, 0, 5, 6, 7, 8]
  assert move_up([1, 4, 2, 3, 0, 5, 6, 7, 8]) == [1, 4, 2, 3, 7, 5, 6, 0, 8]
  assert move_up([0, 1, 2, 3, 4, 5, 6, 7, 8]) == None
output: def move_up(puzzle):
      index = puzzle.index(0)
      if index not in [0, 1, 2]:
          puzzle[index], puzzle[index - 3] = puzzle[index - 3], puzzle[index]
          return puzzle
      return None