chore: fix checkstyle warning

- inner assignments should be avoided

Author: sozelfist

src/main/java/com/thealgorithms/datastructures/trees/SplayTree.java

@@ -33,7 +33,8 @@ private static class Node {
 
         Node(int key) {
             this.key = key;
-            left = right = null;
+            left = null;
+            right = null;
         }
     }