[mypy] fix type annotations in `data_structures/queue/circular_queue_linked_list.py` (#5749)

* [mypy] fix type annotations in circular_queue_linked_list

* Remove 10 blank lines
Co-authored-by: NChristian Clauss <cclauss@me.com>

data_structures/queue/circular_queue_linked_list.py

 # Implementation of Circular Queue using linked lists
 # https://en.wikipedia.org/wiki/Circular_buffer
 
+from __future__ import annotations
+
 from typing import Any
 
 
@@ -18,8 +20,8 @@ class CircularQueueLinkedList:
     """
 
     def __init__(self, initial_capacity: int = 6) -> None:
-        self.front = None
-        self.rear = None
+        self.front: Node | None = None
+        self.rear: Node | None = None
         self.create_linked_list(initial_capacity)
 
     def create_linked_list(self, initial_capacity: int) -> None:
@@ -27,7 +29,7 @@ class CircularQueueLinkedList:
         self.front = current_node
         self.rear = current_node
         previous_node = current_node
-        for i in range(1, initial_capacity):
+        for _ in range(1, initial_capacity):
             current_node = Node()
             previous_node.next = current_node
             current_node.prev = previous_node
@@ -49,9 +51,14 @@ class CircularQueueLinkedList:
         >>> cq.is_empty()
         True
         """
-        return self.front == self.rear and self.front.data is None
 
-    def first(self) -> Any:
+        return (
+            self.front == self.rear
+            and self.front is not None
+            and self.front.data is None
+        )
+
+    def first(self) -> Any | None:
         """
         Returns the first element of the queue
         >>> cq = CircularQueueLinkedList()
@@ -74,7 +81,7 @@ class CircularQueueLinkedList:
         'b'
         """
         self.check_can_perform_operation()
-        return self.front.data
+        return self.front.data if self.front else None
 
     def enqueue(self, data: Any) -> None:
         """
@@ -92,11 +99,13 @@ class CircularQueueLinkedList:
            ...
         Exception: Empty Queue
         """
+        if self.rear is None:
+            return
+
         self.check_is_full()
-        if self.is_empty():
-            self.rear.data = data
-        else:
+        if not self.is_empty():
             self.rear = self.rear.next
+        if self.rear:
             self.rear.data = data
 
     def dequeue(self) -> Any:
@@ -117,6 +126,8 @@ class CircularQueueLinkedList:
         Exception: Empty Queue
         """
         self.check_can_perform_operation()
+        if self.rear is None or self.front is None:
+            return
         if self.front == self.rear:
             data = self.front.data
             self.front.data = None
@@ -133,15 +144,15 @@ class CircularQueueLinkedList:
             raise Exception("Empty Queue")
 
     def check_is_full(self) -> None:
-        if self.rear.next == self.front:
+        if self.rear and self.rear.next == self.front:
             raise Exception("Full Queue")
 
 
 class Node:
     def __init__(self) -> None:
-        self.data = None
-        self.next = None
-        self.prev = None
+        self.data: Any | None = None
+        self.next: Node | None = None
+        self.prev: Node | None = None
 
 
 if __name__ == "__main__":