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

applications/utilities/mesh/manipulation/refineMesh/refineMesh.C

@@ -485,7 +485,7 @@ int main(int argc, char *argv[])
     {
         const labelList& added = oldToNew[oldCellI];
 
-        if (added.size() > 0)
+        if (added.size())
         {
             forAll(added, i)
             {