private Boolean readOptional(Element spec) {
        String optional = spec.getAttribute("optional");
        return (optional.isEmpty()) ?
                null :
                Boolean.parseBoolean(optional);
    }
Builds federation options from a federations options element
@author Tony Vaagenes
config-model/src/main/java/com/yahoo/vespa/model/builder/xml/dom/chains/search/FederationOptionsBuilder.java