- use STRLENOF, looks cleaner

- use STRLENOF, looks cleaner
- in case a value is incorrectly empty (technically, it's null because
  that's how it's returned by the stringification function), make enough
  room to add the error string "(null)"
- in case a value is legitimately empty (see above), use it accordingly;
  for example, a filter "(entryDN=)" is valid, so it should be possible
  to stringify it like that.