Added Null Object pattern.

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

@@ -0,0 +1,33 @@
+package com.iluwatar;
+
+/**
+ * 
+ * Null Object implementation for binary tree node.
+ *
+ */
+public class NullNode implements Node {
+
+	@Override
+	public int getTreeSize() {
+		return 0;
+	}
+
+	@Override
+	public Node getLeft() {
+		return null;
+	}
+
+	@Override
+	public Node getRight() {
+		return null;
+	}
+
+	@Override
+	public String getName() {
+		return null;
+	}
+
+	@Override
+	public void walk() {
+	}
+}