BUG: #3846 fixes crash when trying to use label map seeding

With the change to VTKv6 some member variables can be null, probably
due to the tensor pipeline not being correctly initialized.

This avoids the crash, but issue #3846 is not fixed (tractography
does not work).

From: Steve Pieper <pieper@bwh.harvard.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23680 3bd1e089-480b-0410-8dfb-8563597acbee