Simplify checking of container (List/HashTable, strings) sizes

- can use 'XX.empty()' instead of 'XX.size() == 0', 'XX.size() < 1' or
  'XX.size() <= 0' or for simpler coding.
  It also has the same number of characters as '!XX.size()' and /might/ be
  more readable

- many size checking had 'XX.size() > 0', 'XX.size() != 0', or 'XX.size() >= 1'
  when a simple 'XX.size()' suffices

src/meshTools/sets/pointSources/nearestToPoint/nearestToPoint.C

@@ -62,7 +62,7 @@ void Foam::nearestToPoint::combine(topoSet& set, const bool add) const
     {
         const pointField& pts = mesh_.points();
 
-        if (pts.size() > 0)
+        if (pts.size())
         {
             label minPointI = 0;
             scalar minDistSqr = magSqr(pts[minPointI] - points_[pointI]);