fix Prevent vtkPositionOrientationStream from returning nothing

The vtkPositionOrientationStream was returning nothing is case no new data were available instead of returning the previous data. When visualized via a Spreadsheet, this lead to blicking data (nothing / data). Also updating the last data requested, should be update just after the data are requested, and not in a function which is also call in other places.

@melanie.carriere PTAL