default Set<String> getPossibleUserAttributeNames(final PersonAttributeDaoFilter filter) {
        return new LinkedHashSet<>();
    }
Gets a {@link Set} of attribute names that may be returned for an IPersonAttributes. The names returned represent all
possible attributes names for the {@link PersonAttributes} objects returned by the get methods. If the dao doesn't have a
way to know all possible attribute names this method should return {@code null}.
<br>
Returns an immutable {@link Set}.

@param filter the filter
@return A {@link Set} of possible attribute names for user queries.
api/cas-server-core-api-authentication/src/main/java/org/apereo/cas/authentication/principal/attribute/PersonAttributeDao.java