PATCH: qMRMLLabelComboBox default value is -1 until set to > 0.

Pending Slicer fix: https://github.com/jcfr/Slicer/commit/8e01bd3558be72080044f2a2ae41bc46ced8abf4

Change-Id: I75f12d976dca4e71bdc7b6da6292db38700dfa05
Merge topic 'workaround-slicer-qmrmllabelcombobox-bug'

6b999b6 PATCH: qMRMLLabelComboBox default value is -1 until set to > 0.