docs: @Nullable
    public static String getUserDisplayName (@Nullable final String sUserID, @Nonnull final Locale aDisplayLocale)
    {
      if (StringHelper.hasNoText (sUserID))
        return getGuestUserDisplayName (aDisplayLocale);
  
      final IUser aUser = PhotonSecurityManager.getUserMgr ().getUserOfID (sUserID);
      return aUser == null ? sUserID : getUserDisplayName (aUser, aDisplayLocale);
    }