[mypy] Fix type annotations in data_structures/binary_tree (#5518)

* fix: fix mypy errors

Update binary_search_tree `arr` argument to be typed as a list
within `find_kth_smallest` function

Update return type of `merge_two_binary_trees` as both inputs can
be None which means that a None type value can be returned from
this function

* updating DIRECTORY.md

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>

data_structures/binary_tree/binary_search_tree.py

@@ -151,7 +151,7 @@ class BinarySearchTree:
 
     def find_kth_smallest(self, k: int, node: Node) -> int:
         """Return the kth smallest element in a binary search tree"""
-        arr = []
+        arr: list = []
         self.inorder(arr, node)  # append all values to list using inorder traversal
         return arr[k - 1]