private static Optional<String> stringAttribute(String attributeName, Element tag, boolean ignoreBlanks) {
        String value = tag.getAttribute(attributeName);
        return Optional.of(value).filter(s -> (tag.getAttributeNode(attributeName) != null && ! ignoreBlanks || ! s.isBlank()));
    }
Returns the given non-blank attribute of tag as a string, if any
config-model-api/src/main/java/com/yahoo/config/application/api/xml/DeploymentSpecXmlReader.java