- adjust nullptr checks to discourage flip-flop when confronted with
  multiple null values.

     Old:   (a && b) ? (*a < *b) : bool(a);
     New:   (a && b) ? (*a < *b) : !b;

  comparing (non-null < null) and (null < non-null) behaves
  identically, but comparing (null < null) now tests as true
  (ie, already sorted) whereas before it would have been false
  (ie, needs a swap)

- add UPtrList trimTrailingNull(), which reduces the effective
  (addressable) list size to ignore any trailing null pointers, but
  without reallocation. This is particularly useful when creating a
  UPtrList list view. For example,

     UPtrList<some_iterator> validValues(container.size());

     ...Loop to add valid entries, by some criteria...

     // Shorten list to hide null entries
     validValues.trimTrailingNull();

   This list view now only needs a single allocation, whereas using
   a resize (as was previously necessary) could invoke a second
   allocation, as well as recopying.