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/vtkMesh.C

@@ -47,7 +47,7 @@ Foam::vtkMesh::vtkMesh
     subsetter_(baseMesh),
     setName_(setName)
 {
-    if (setName.size() > 0)
+    if (setName.size())
     {
         // Read cellSet using whole mesh
         cellSet currentSet(baseMesh_, setName_);
@@ -71,7 +71,7 @@ Foam::polyMesh::readUpdateState Foam::vtkMesh::readUpdate()
 
         topoPtr_.clear();
 
-        if (setName_.size() > 0)
+        if (setName_.size())
         {
             Info<< "Subsetting mesh based on cellSet " << setName_ << endl;