Write a function to find the total number of leaf nodes in a binary tree. A node is described as a leaf node if it doesn't have any children. If there are no leaf nodes, return 0.
1. Recursively traverse the tree.
2. At each node, determine if its left and right child nodes are null.
2a. If true, return 1.
2b. Otherwise, find number of leaves in its left and right subtrees (recursively).
3. Return the sum of the number of leaves in the left and the right subtrees.