BUG: XMLMultiBlockDataWriter always pointed to vtp files.
BUG: XMLMultiBlockDataWriter always pointed to vtp files.

Because of a reverted test for empty vector, the type
of pieces on processors would always be 0 (vtp).

gram526/VTK,keithroe/vtkoptix,demarle/VTK,msmolens/VTK,candy7393/VTK,sumedhasingla/VTK,SimVascular/VTK,msmolens/VTK,gram526/VTK,candy7393/VTK,keithroe/vtkoptix,gram526/VTK,jmerkow/VTK,candy7393/VTK,demarle/VTK,mspark93/VTK,sumedhasingla/VTK,demarle/VTK,msmolens/VTK,mspark93/VTK,candy7393/VTK,sankhesh/VTK,jmerkow/VTK,sumedhasingla/VTK,gram526/VTK,msmolens/VTK,gram526/VTK,sankhesh/VTK,candy7393/VTK,candy7393/VTK,msmolens/VTK,keithroe/vtkoptix,sumedhasingla/VTK,demarle/VTK,sankhesh/VTK,keithroe/vtkoptix,jmerkow/VTK,mspark93/VTK,sumedhasingla/VTK,mspark93/VTK,sankhesh/VTK,demarle/VTK,sumedhasingla/VTK,sankhesh/VTK,gram526/VTK,msmolens/VTK,sumedhasingla/VTK,mspark93/VTK,jmerkow/VTK,SimVascular/VTK,candy7393/VTK,msmolens/VTK,SimVascular/VTK,mspark93/VTK,keithroe/vtkoptix,sankhesh/VTK,msmolens/VTK,keithroe/vtkoptix,SimVascular/VTK,SimVascular/VTK,jmerkow/VTK,mspark93/VTK,jmerkow/VTK,jmerkow/VTK,SimVascular/VTK,demarle/VTK,gram526/VTK,mspark93/VTK,gram526/VTK,keithroe/vtkoptix,demarle/VTK,keithroe/vtkoptix,SimVascular/VTK,sankhesh/VTK,demarle/VTK,jmerkow/VTK,candy7393/VTK,sumedhasingla/VTK,sankhesh/VTK,SimVascular/VTK