fix: handle None node in list helpers

Author: AhmadHmedann

Sprint-2/implement_lru_cache/lru_cache.py

@@ -13,6 +13,8 @@ def __init__(self, limit):
         self.cache = {}
 
     def add_to_head(self, node):
+      if node == None:
+          return
       if self.head is None:
         self.head = node
         self.tail = node