fix error in SinglyLinkedList head method

Author: BekzodIsakov

Data-Structures/Linked-List/SinglyLinkedList.js

@@ -40,7 +40,7 @@ class LinkedList {
 
   // Returns the head
   head () {
-    return this.headNode?.data || null
+    return this.headNode?.data ?? null
   }
 
   // Returns the tail

Data-Structures/Linked-List/test/SinglyLinkedList.test.js

@@ -148,6 +148,9 @@ describe('SinglyLinkedList', () => {
 
     list.addFirst(30)
     expect(list.head()).toBe(30)
+
+    list.addFirst(0)
+    expect(list.head()).toBe(0)
   })
 
   it('Check tail', () => {