[LinkedList] Simplify code style for Merge Two Sorted Lists

Author: Yi Gu

LinkedList/MergeTwoSortedLists.swift

@@ -33,11 +33,7 @@ class MergeTwoSortedLists {
             node = node.next!
         }
 
-        if l1 != nil {
-            node.next = l1
-        } else if l2 != nil {
-            node.next = l2
-        }
+        node.next = l1 ?? l2
 
         return dummy.next
     }