default boolean isEmpty() {
		return value() == null && childCount() == 0 && parent().isEmpty();
	}
A tree is considered <em>empty</em> if it's {@link #value()} is
{@code null} and has no children and parent. A newly created tree node
with no value is <em>empty</em>.

{@snippet lang="java":
final Tree<String, ?> tree = TreeNode.of();
assert tree.isEmpty();
}

@since 7.0

@return {@code true} if {@code this} tree is empty, {@code false}
         otherwise
jenetics.ext/src/main/java/io/jenetics/ext/util/Tree.java