handle tables with no row labels

More gracefully handle the situation where the input table does
not contain a vtkStringArray column naming the rows.  Previously,
this class assumed that the first column was a vtkStringArray,
leading to a segmentation fault when it was not.

Change-Id: Ie0aeca049f23885b85a51d6c8b2a43dbc0e1afcd
Merge topic 'no_names_heatmap' into master

270a6e4d handle tables with no row labels