ENH: add 'filtered' polyMesh regionName() method

- in various situations with mesh regions it is also useful to
  filter out or remove the defaultRegion name (ie, "region0").

  Can now do that conveniently from the polyMesh itself or as a static
  function. Simply use this

      const word& regionDir = polyMesh::regionName(regionName);

  OR  mesh.regionName()

  instead of

      const word& regionDir =
      (
           regionName != polyMesh::defaultRegion
         ? regionName
         : word::null
      );

  Additionally, since the string '/' join operator filters out empty
  strings, the following will work correctly:

      (polyMesh::regionName(regionName)/polyMesh::meshSubDir)

      (mesh.regionName()/polyMesh::meshSubDir)

applications/utilities/preProcessing/createZeroDirectory/createZeroDirectory.C

@@ -145,7 +145,7 @@ void createFieldFiles
 
         fileName regionPath = "/";
 
-        if (regionName != word::null)
+        if (!regionName.empty())
         {
             regionPath += regionName + '/';
         }
@@ -294,7 +294,7 @@ int main(int argc, char *argv[])
     {
         const word& regionName = solver.regionName(regionI);
 
-        if (regionName == word::null)
+        if (regionName.empty())
         {
             Info<< "Region: " << polyMesh::defaultRegion << " (default)"
                 << endl;