ENH: code adjustments for updated NullObject

- can now safely use labelList::null() instead of emptyLabelList for
  return values. No special treatment required for lists.

  Possible replacements:

      if (notNull(list) && list.size())  ->  if (list.size())
      if (isNull(list) || list.empty())  ->  if (list.empty())

  The receiver may still wish to handle differently to distinguish
  between a null list and an empty list, but no additional special
  protection is required when obtaining sizes, traversing, outputting
  etc.