docs: @Nullable
    public static String getUserDisplayName (@Nullable final IUser aUser, @Nonnull final Locale aDisplayLocale)
    {
      ValueEnforcer.notNull (aDisplayLocale, "DisplayLocale");
      if (aUser == null)
        return getGuestUserDisplayName (aDisplayLocale);
  
      String ret = aUser.getDisplayName ();
      if (StringHelper.hasNoText (ret))
        ret = aUser.getLoginName ();
      return ret;
    }