public static Optional<String> getOptionalChildValue(Element parent, String childName) {
        Element child = XML.getChild(parent, childName);
        if (child == null) return Optional.empty();
        if (child.getFirstChild() == null) return Optional.empty();
        return Optional.ofNullable(child.getFirstChild().getNodeValue());
    }
Creates a new XML document builder.

@return a new DocumentBuilder instance, or null if we fail to get one.
config-model/src/main/java/com/yahoo/config/model/builder/xml/XmlHelper.java