private <T> T getWithFallback(Element parent, Element fallbackParent, String tagName, String attributeName,
                                  Function<String, T> mapper, T fallbackValue) {
        Element element = XML.getChild(parent, tagName);
        if (element == null) element = XML.getChild(fallbackParent, tagName);
        if (element == null) return fallbackValue;
        String attribute = element.getAttribute(attributeName);
        return attribute.isBlank() ? fallbackValue : mapper.apply(attribute);
    }
Returns true if the given value is "true", or if it is missing
config-model-api/src/main/java/com/yahoo/config/application/api/xml/DeploymentSpecXmlReader.java