input: Write a python function to drop all leaf nodes from a binary tree.
  assert dropLeafNodes(Node(1, Node(2), Node(3))).left is None and dropLeafNodes(Node(1, Node(2), Node(3))).right is None
  assert dropLeafNodes(Node(1, Node(2, Node(4), Node(5)), Node(3))).left.left is None and dropLeafNodes(Node(1, Node(2, Node(4), Node(5)), Node(3))).left.right is None
  assert dropLeafNodes(Node(1, Node(2, None, Node(3)), Node(4, Node(5), None))).right.left is None
output: class Node:
      def __init__(self, val=0, left=None, right=None):
          self.val = val
          self.left = left
          self.right = right
  
  def dropLeafNodes(root):
      if root is None:
          return None
      if root.left is None and root.right is None:
          return None
      root.left = dropLeafNodes(root.left)
      root.right = dropLeafNodes(root.right)
      return root