public Optional<UserInfo> getByUuid(final String userUuid) {
        return NullSafe.isBlankString(userUuid)
                ? Optional.empty()
                : cacheByUuid.get(userUuid);
    }
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