code: @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);
    }
docs: Get the display name of the user.
  
  @param sUserID
  User ID. May be <code>null</code>.
  @param aDisplayLocale
  The display locale to be used.
  @return The "guest" text if no user ID was provided, the display name of
  the user if a valid user ID was provided or the ID of the user if
  an invalid user was provided.