Code cleanup (#1461)

* Code cleanup

* Fix flux tests

* Fix checkstyle errors

* Fix compile error

null-object/src/main/java/com/iluwatar/nullobject/NullNode.java

@@ -30,7 +30,7 @@ package com.iluwatar.nullobject;
  */
 public final class NullNode implements Node {
 
-  private static NullNode instance = new NullNode();
+  private static final NullNode instance = new NullNode();
 
   private NullNode() {
   }