input: Write a function to drop all leaf nodes from a Huffman tree. The function should take the root node of the tree as input and return the root of the modified tree after removing all leaf nodes. If the input tree is empty (i.e., the root is None), the function should return None.
  
  assert dropLeafNodes((1, left=(2, left=4, right=5), right=(3, left=6, right=7)))==(1, left=2, right=3)
  assert dropLeafNodes(None)==None
  assert dropLeafNodes((1, left=2, right=None))==1
  assert dropLeafNodes((1, left=2, right=3))==1
  assert dropLeafNodes((1, left=None, right=None))==None
output: def dropLeafNodes(root):
      if root is None:
          return None
      if isinstance(root, tuple):
          value, left, right = root
          if left is None and right is None:
              return None
          left = dropLeafNodes(left)
          right = dropLeafNodes(right)
          return (value, left, right)
      return root