restore "missing rows" behavior to vtkHeatmapItem

Properly handle the case where the data table does not contain a row
for every leaf node in the tree.

This commit fixes a regression that probably occurred around commit
3ea804: when vtkTreeHeatmapItem was refactored into separate classes
for the dendrogram and the heatmap.

Change-Id: I07c5de340c7a7845cf2172c0cd7f334e3e46b932
Merge topic 'support_missing_data' into master

e2711892 restore "missing rows" behavior to vtkHeatmapItem