Fix type hint

Author: Hardvan

data_structures/linked_list/merge_sort_linked_list.py

@@ -20,7 +20,7 @@ def __init__(self, data: int) -> None:
         self.next: Node | None = None
 
 
-def get_middle(head: Node) -> Node:
+def get_middle(head: Node | None) -> Node | None:
     """
     Find the middle node of the linked list using the slow and fast pointer technique.