code: public static YamlNode nullToNode(YamlNode value) {
          return Optional.ofNullable(value).orElseGet(YamlNullNode::instance);
      }
docs: Checks if {@code value} is {@code null} and returns {@code value} or a
  instance of {@link YamlNullNode}.
  
  @param value the node to test
  
  @return {@code value} if not {@code null}, else a {@link YamlNullNode}