code: @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;
    }
docs: Get the display name of the user. If no display name is present (because
  first name and last name are empty), the login name is returned.
  
  @param aUser
  User. May be <code>null</code>.
  @param aDisplayLocale
  The display locale to be used to resolve the "Guest" text if the
  passed user is <code>null</code>.
  @return Never <code>null</code>. Either the display name or the login name
  of the user.