public Optional<UserInfo> getByRef(final UserRef userRef) {
        Objects.requireNonNull(userRef, "User ref is null");
        return cacheByUuid.get(userRef.getUuid());
    }
Gets a user/group by their stroom user UUID if they exist.
stroom-security/stroom-security-impl/src/main/java/stroom/security/impl/UserInfoCache.java