code: public static String sort(String str) {
          if (N.isNullOrEmpty(str)) {
              return str;
          }
  
          final char[] chs = str.toCharArray();
          Array.sort(chs);
          return StringUtil.newString(chs, true);
      }
docs: Returns a new sorted String if the specified {@code str} is not null or empty, otherwise the specified {@code str} is returned.
  
  @param str
  @return