public Mono<UserSessionsStatus> getUserSessionsStatus(@NotNull Long userId) {
        try {
            Validator.notNull(userId, "userId");
        } catch (ResponseException e) {
            return Mono.error(e);
        }
        if (cacheUserSessionsStatus) {
            UserSessionsStatus sessionsStatus = userIdToStatusCache.getIfPresent(userId);
            if (sessionsStatus != null) {
                return Mono.just(sessionsStatus);
            }
        }
        return fetchUserSessionsStatus(userId);
    }
@return {@link UserStatus#OFFLINE} instead of MonoEmpty for an offline user
turms-server-common/src/main/java/im/turms/server/common/domain/session/service/UserStatusService.java