Fix crash when vtkPlotPoints has no Axis
Fix crash when vtkPlotPoints has no Axis

Sometimes vtkPlotPoints is not added as a plot into a chart.
This is the case vtkPlotFunctionalBag. Therefore when calling Update(),
there can be no Axis.

Change-Id: I5ccd256173831e31b048b732a6021286266e530f

jmerkow/VTK,demarle/VTK,hendradarwin/VTK,msmolens/VTK,jmerkow/VTK,hendradarwin/VTK,ashray/VTK-EVM,hendradarwin/VTK,ashray/VTK-EVM,SimVascular/VTK,sankhesh/VTK,SimVascular/VTK,keithroe/vtkoptix,jmerkow/VTK,sankhesh/VTK,SimVascular/VTK,sankhesh/VTK,mspark93/VTK,sankhesh/VTK,demarle/VTK,mspark93/VTK,mspark93/VTK,gram526/VTK,keithroe/vtkoptix,keithroe/vtkoptix,demarle/VTK,jmerkow/VTK,sumedhasingla/VTK,mspark93/VTK,msmolens/VTK,demarle/VTK,msmolens/VTK,candy7393/VTK,hendradarwin/VTK,demarle/VTK,hendradarwin/VTK,jmerkow/VTK,SimVascular/VTK,ashray/VTK-EVM,keithroe/vtkoptix,gram526/VTK,ashray/VTK-EVM,jmerkow/VTK,ashray/VTK-EVM,gram526/VTK,gram526/VTK,hendradarwin/VTK,keithroe/vtkoptix,SimVascular/VTK,sankhesh/VTK,msmolens/VTK,sankhesh/VTK,demarle/VTK,sumedhasingla/VTK,sankhesh/VTK,gram526/VTK,gram526/VTK,demarle/VTK,sumedhasingla/VTK,sumedhasingla/VTK,keithroe/vtkoptix,candy7393/VTK,ashray/VTK-EVM,candy7393/VTK,demarle/VTK,candy7393/VTK,ashray/VTK-EVM,keithroe/vtkoptix,gram526/VTK,candy7393/VTK,mspark93/VTK,ashray/VTK-EVM,msmolens/VTK,SimVascular/VTK,SimVascular/VTK,msmolens/VTK,sumedhasingla/VTK,sankhesh/VTK,SimVascular/VTK,msmolens/VTK,msmolens/VTK,keithroe/vtkoptix,mspark93/VTK,gram526/VTK,sumedhasingla/VTK,candy7393/VTK,sumedhasingla/VTK,jmerkow/VTK,sumedhasingla/VTK,candy7393/VTK,jmerkow/VTK,mspark93/VTK,hendradarwin/VTK,mspark93/VTK,candy7393/VTK