BUG #14273: Don't segfault when mapper has empty input.
BUG #14273: Don't segfault when mapper has empty input.

vtkPainterPolyDataMapper segfaults when input is empty. That's unnecessary. This
check avoids that segfault.

Change-Id: Ie572e3963c85d7c71abad644eebc4313ed93992d

sankhesh/VTK,keithroe/vtkoptix,SimVascular/VTK,sumedhasingla/VTK,berendkleinhaneveld/VTK,mspark93/VTK,SimVascular/VTK,msmolens/VTK,gram526/VTK,berendkleinhaneveld/VTK,ashray/VTK-EVM,gram526/VTK,msmolens/VTK,keithroe/vtkoptix,sankhesh/VTK,demarle/VTK,sumedhasingla/VTK,SimVascular/VTK,ashray/VTK-EVM,johnkit/vtk-dev,SimVascular/VTK,msmolens/VTK,SimVascular/VTK,biddisco/VTK,hendradarwin/VTK,biddisco/VTK,gram526/VTK,hendradarwin/VTK,johnkit/vtk-dev,demarle/VTK,msmolens/VTK,mspark93/VTK,biddisco/VTK,keithroe/vtkoptix,ashray/VTK-EVM,demarle/VTK,ashray/VTK-EVM,SimVascular/VTK,sumedhasingla/VTK,candy7393/VTK,berendkleinhaneveld/VTK,candy7393/VTK,mspark93/VTK,hendradarwin/VTK,ashray/VTK-EVM,jmerkow/VTK,msmolens/VTK,candy7393/VTK,gram526/VTK,biddisco/VTK,johnkit/vtk-dev,berendkleinhaneveld/VTK,msmolens/VTK,gram526/VTK,sumedhasingla/VTK,sumedhasingla/VTK,mspark93/VTK,jmerkow/VTK,sumedhasingla/VTK,hendradarwin/VTK,demarle/VTK,gram526/VTK,sankhesh/VTK,hendradarwin/VTK,berendkleinhaneveld/VTK,sankhesh/VTK,keithroe/vtkoptix,gram526/VTK,msmolens/VTK,keithroe/vtkoptix,jmerkow/VTK,johnkit/vtk-dev,demarle/VTK,candy7393/VTK,candy7393/VTK,biddisco/VTK,johnkit/vtk-dev,ashray/VTK-EVM,sankhesh/VTK,hendradarwin/VTK,biddisco/VTK,johnkit/vtk-dev,mspark93/VTK,jmerkow/VTK,SimVascular/VTK,demarle/VTK,sankhesh/VTK,mspark93/VTK,candy7393/VTK,biddisco/VTK,sankhesh/VTK,mspark93/VTK,sumedhasingla/VTK,berendkleinhaneveld/VTK,demarle/VTK,johnkit/vtk-dev,gram526/VTK,SimVascular/VTK,hendradarwin/VTK,candy7393/VTK,jmerkow/VTK,candy7393/VTK,jmerkow/VTK,mspark93/VTK,demarle/VTK,msmolens/VTK,ashray/VTK-EVM,jmerkow/VTK,ashray/VTK-EVM,keithroe/vtkoptix,keithroe/vtkoptix,jmerkow/VTK,keithroe/vtkoptix,sankhesh/VTK,sumedhasingla/VTK,berendkleinhaneveld/VTK