Fix freeze when running in parallel.

vtkPConvertSelection would not execute symmetrically in parallel as it
was skipping RequestData() if the dataset was empty. That no longer can
work since ExtractSelection can result in parallel communication. This
fixes that problem by not skipping execution when data is empty on the
current rank.

Change-Id: I5acc06cd2dbe5266ebb9165bc1b8388a738c00c8
Merge topic '9808-bugfix' into next

cf238bb7 Fix freeze when running in parallel.