BUG #14273: Don't segfault when mapper has empty input.

vtkPainterPolyDataMapper segfaults when input is empty. That's unnecessary. This
check avoids that segfault.

Change-Id: Ie572e3963c85d7c71abad644eebc4313ed93992d
Merge topic 'fix_segfault_on_invalid_input' into master

f694dbf2 BUG #14273: Don't segfault when mapper has empty input.