📍Use lombok, reformat, and optimize the code (#1560)

* Use lombok, reformat, and optimize the code

* Fix merge conflicts and some sonar issues

Co-authored-by: va1m <va1m@email.com>

null-object/src/test/java/com/iluwatar/nullobject/TreeTest.java

@@ -88,7 +88,7 @@ public class TreeTest {
    * Node#getTreeSize()} of 7 {@link Node}s in total.
    */
   @Test
-  public void testTreeSize() {
+  void testTreeSize() {
     assertEquals(7, TREE_ROOT.getTreeSize());
   }
 
@@ -96,7 +96,7 @@ public class TreeTest {
    * Walk through the tree and verify if every item is handled
    */
   @Test
-  public void testWalk() {
+  void testWalk() {
     TREE_ROOT.walk();
 
     assertTrue(appender.logContains("root"));
@@ -110,7 +110,7 @@ public class TreeTest {
   }
 
   @Test
-  public void testGetLeft() {
+  void testGetLeft() {
     final var level1 = TREE_ROOT.getLeft();
     assertNotNull(level1);
     assertEquals("level1_a", level1.getName());
@@ -130,7 +130,7 @@ public class TreeTest {
   }
 
   @Test
-  public void testGetRight() {
+  void testGetRight() {
     final var level1 = TREE_ROOT.getRight();
     assertNotNull(level1);
     assertEquals("level1_b", level1.getName());