BUG #0015137: Fixed label format.

Setting the label format to empty by default ensures that VTK picks a
label format based on the data type. It's better to use that that some
hard-coded default. That addresses the reported BUG.

Change-Id: I8be35a176258cfe1d4c3ef94e4efde0d9588f432
Merge topic '15137_fix_label_format' into next

7ea8415e BUG #0015137: Fixed label format.