input: Write a function to remove all leaf nodes from a given Huffman tree. The function should take the head of the Huffman tree as input and return the modified tree without any leaf nodes. If the input tree is empty or the head is None, the function should return None.
  
  assert dropLeafNodes(None) == None
  assert dropLeafNodes(HuffTree(1, HuffTree(2, HuffTree(4, None, None), HuffTree(5, None, None)), HuffTree(3, HuffTree(6, None, None), HuffTree(7, None, None)))) == HuffTree(1, HuffTree(2, None, None), HuffTree(3, None, None))
  assert dropLeafNodes(HuffTree(2, HuffTree(4, None, None), HuffTree(5, None, None))) == None
  assert dropLeafNodes(HuffTree(3)) == None
output: def dropLeafNodes(head):
      if head is None:
          return None
      if head.left is None and head.right is None:
          return None
      head.left = dropLeafNodes(head.left)
      head.right = dropLeafNodes(head.right)
      if head.left is None and head.right is None:
          return None
      return head