input: Write a function to implement a Python function that takes a Huffman tree and drop all its leaf nodes.
  assert dropLeafNodes({'value': 1, 'left': {'value': 2, 'left': {'value': 4}, 'right': {'value': 5}}, 'right': {'value': 3, 'left': {'value': 6}, 'right': {'value': 7}}})=={'value': 1, 'left': {'value': 2}, 'right': {'value': 3}}
  assert dropLeafNodes({'value': 1})==None
  assert dropLeafNodes(None)==None
output: def dropLeafNodes(tree):
      if tree is None:
          return None
      if 'left' not in tree and 'right' not in tree:
          return None
      tree['left'] = dropLeafNodes(tree.get('left'))
      tree['right'] = dropLeafNodes(tree.get('right'))
      return tree