code: public String extractNodeValue(@Nullable Node parentNode, Iterable<String> xPath) {
      Node node = extractNode(parentNode, xPath);
      if (node == null || node.getFirstChild() == null) {
        return null;
      }
      return node.getFirstChild().getNodeValue();
    }
docs: Convenience method for getting the value of a node, handling the case where the node does not
  exist.
  
  @return the value of the node at {@code xPath} if it exists, else {@code null}.