input: Write a function to implement a function in Python that takes a Huffman tree and drop all its leaf nodes.
  assert dropLeafNodes([1, [2, [4, None, None], [5, None, None]], [3, [6, None, None], [7, None, None]]])==[1, [2, None, None], [3, None, None]]
  assert dropLeafNodes([10, [20, None, None], [30, [40, None, None], [50, None, None]]])==[10, None, [30, None, None]]
  assert dropLeafNodes([])==None
  assert dropLeafNodes([100, None, None])==None
output: def dropLeafNodes(tree):
      if not tree:
          return None
      if tree[1] is None and tree[2] is None:
          return None
      left = dropLeafNodes(tree[1])
      right = dropLeafNodes(tree[2])
      return [tree[0], left, right]