code: public static AbstractVariable stringToVariable(String s) {
          return s == null ? Null.instance : new OctetString(s);
      }
docs: Convert a server-side string into either an OctetString (if the string is non-null) or Null.instance ("the value of this
  OID is null") for transmission. This is so the client side can tell the difference between real nulls and empty values.
  
  @param s
  @return