public static AuditActor user(@Nonnull String username) {
        if (isNullOrEmpty(username)) {
            throw new IllegalArgumentException("username must not be null or empty");
        }
        return new AutoValue_AuditActor(URN_GRAYLOG_USER + username);
    }
@Test(expected = IllegalArgumentException.class)
    public void testEmptyUser() {
        AuditActor.user("");
    }