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
         )
         {

applications/utilities/postProcessing/dataConversion/foamToVTK/readFields.C

@@ -58,7 +58,7 @@ void readFields
         ++iter
     )
     {
-        if (!selectedFields.size() || selectedFields.found(iter()->name()))
+        if (selectedFields.empty() || selectedFields.found(iter()->name()))
         {
             fields.set
             (

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;
 

applications/utilities/postProcessing/dataConversion/foamToVTK/vtkMesh.H

@@ -105,13 +105,13 @@ public:
             //- Check if running subMesh
             bool useSubMesh() const
             {
-                return setName_.size() > 0;
+                return setName_.size();
             }
 
             //- topology
             const vtkTopo& topo() const
             {
-                if (!topoPtr_.valid())
+                if (topoPtr_.empty())
                 {
                     topoPtr_.reset(new vtkTopo(mesh()));
                 }