default Set<String> getAvailableQueryAttributes(final PersonAttributeDaoFilter filter) {
        return new LinkedHashSet<>();
    }
Gets a {@link Set} of attribute names that this implementation knows how to use in a query. The names returned
represent all possible names for query attributes for this implmenentation. If the dao doesn't have a way to know
all possible query attribute names this method should return {@code null}
<br>
Returns an immutable {@link Set}.

@param filter the filter
@return The set of attributes that can be used to query for user ids in this dao, null if the set is unknown.
api/cas-server-core-api-authentication/src/main/java/org/apereo/cas/authentication/principal/attribute/PersonAttributeDao.java