BUG: Fix invalid call to GetDataType() when vtkPoints is NULL

When desired output precision is DEFAULT_PRECISION and the input
is empty, this prevents GetDataType() from being called on a
NULL vtkPoints instance.

Change-Id: I10e1fd8db008c015f1c96ece33007e35273329c4
Merge topic 'fix-desired-output-precision-empty-input' into master

399b31f7 BUG: Fix invalid call to GetDataType() when vtkPoints is NULL