make vtkTreeHeatmapItem tolerate missing input

Avoid segmentation faults when SetTree or SetTable are called with
NULL arguments.  Update GetBounds so that it no longer assumes both
a tree and a table have been loaded.

Change-Id: I72ad76a996136aeeaf0c06d25ebe70078d1075c5
Merge topic 'fix_heatmap_crash' into master

76d6ab2e make vtkTreeHeatmapItem tolerate missing input