vtkChartXY didn't update the selection if an empty vtkSelection was
passed. It should have cleared the selection, since it's now being set
to 'empty'. Fixed that.

This fixes the issue in ParaView where is the selection wasn't cleared
when expected (BUG #14935).

Change-Id: I947b13c2b5885c4e182c35013f1fe491d6839c56