BUG #14086: Fix vtkImageAppend when no active scalars are present.

vtkImageAppend failed when the image data didn't have any point-arrays marked as
scalars. Though vtkImageAppend's execution handled that case correctly, the
initialization did not. This patch fixes the initialization code.

Change-Id: I9207646b13038a96a26051fc3aed6d554a55aaf3
Merge topic 'fix-image-append' into master

710ce99d BUG #14086: Fix vtkImageAppend when no active scalars are present.