Adjust checkstyle rules. Make checkstyle fail the build when violations are found. Correct all current checkstyle violations.

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

@@ -11,6 +11,9 @@ public class NodeImpl implements Node {
   private final Node left;
   private final Node right;
 
+  /**
+   * Constructor
+   */
   public NodeImpl(String name, Node left, Node right) {
     this.name = name;
     this.left = left;