Added support for empty Mesh Blueprint coming from Conduit to Catalyst

Catalyst 2.0 used to fail when the simulation code sends a mesh through Conduit with a full Conduit tree that matches the Mesh Blueprint but without any cells or points. This can for instance happen on distributed data. This commit checks the number of cells in the Conduit tree and returns an empty vtkUnstructuredGrid when needed. This commit only fixes the issue for unstructured meshes. This commit also fixes a crash on meshes containing a single cell (rare occasion, but can happen).