Node: may be empty, having no data

* isEmpty()

src/main/java/net/kemitix/node/Node.java

@@ -20,6 +20,13 @@ public interface Node<T> {
      */
     T getData();
 
+    /**
+     * Returns true if the node is empty (has no data).
+     *
+     * @return true is data is null
+     */
+    boolean isEmpty();
+
     /**
      * Fetch the parent node.
      * <p>

src/main/java/net/kemitix/node/NodeItem.java

@@ -50,6 +50,11 @@ public class NodeItem<T> implements Node<T> {
         return data;
     }
 
+    @Override
+    public boolean isEmpty() {
+        return data == null;
+    }
+
     @Override
     public Node<T> getParent() {
         return parent;