Fixed logic to work when an empty vtkSelection is passed.

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
Fix handling of empty selection on ParallelCoordinates.

Same as ac920d1f4a, except this time for vtkChartParallelCoordinates.
The fix exposed a problem with vtkPlotParallelCoordinates where when the
selection was set to NULL, it was never updated after that. This was
incorrect (since for a similar condition in vtkPlotPoints, we create a
new vtkIdTypeArray, when none is present). Fixed that following the
pattern in vtkPlotPoints.

Change-Id: I77cd9d3c6f9f0a616261439fd2db6b2991147d55
Merge topic 'fix_empty_selection' into master

d7aca982 Fix handling of empty selection on ParallelCoordinates.
ac920d1f Fixed logic to work when an empty vtkSelection is passed.