Always write number of groups for extensible GroupItem
A recent change to jsonGroupItem.cxx (MR !1756 (merged)) created an inconsistency.
- The to_json() method only writes the "NumberOfGroups" field if that number is > 0
- The from_json() method now expects the "NumberOfGroups" field for all extensible group items.
The current code logs an error (although it does not appear to be a fatal). This MR removes the error by always writing the number for extensible GroupItem instances.