Fixed most reported issues by SonarCloud.

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

@@ -25,12 +25,15 @@ package com.iluwatar.nullobject;
 
 import org.junit.jupiter.api.Test;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
 /**
  * Application test
  */
-public class AppTest {
+class AppTest {
+
   @Test
-  public void test() {
-    App.main(new String[]{});
+  void shouldExecuteApplicationWithoutException() {
+    assertDoesNotThrow(() -> App.main(new String[]{}));
   }
 }

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

@@ -35,20 +35,20 @@ import static org.junit.jupiter.api.Assertions.assertSame;
  *
  * @author Jeroen Meulemeester
  */
-public class NullNodeTest {
+class NullNodeTest {
 
   /**
    * Verify if {@link NullNode#getInstance()} actually returns the same object instance
    */
   @Test
-  public void testGetInstance() {
+  void testGetInstance() {
     final var instance = NullNode.getInstance();
     assertNotNull(instance);
     assertSame(instance, NullNode.getInstance());
   }
 
   @Test
-  public void testFields() {
+  void testFields() {
     final var node = NullNode.getInstance();
     assertEquals(0, node.getTreeSize());
     assertNull(node.getName());
@@ -56,9 +56,8 @@ public class NullNodeTest {
     assertNull(node.getRight());
   }
 
-  @Test
-  public void testWalk() {
-    NullNode.getInstance().walk();
-  }
+  /**
+   * Removed unnecessary test method for {@link NullNode#walk()} as the method doesn't have an implementation.
+   */
 
 }