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/postProcessing/dataConversion/foamToVTK/foamToVTK.C

@@ -339,7 +339,7 @@ int main(int argc, char *argv[])
         (
             args.options().found("time")
          || args.options().found("latestTime")
-         || cellSetName.size() > 0
+         || cellSetName.size()
          || regionName != polyMesh::defaultRegion
         )
         {