chore: handle null notification summary values

[yashmehrotra]

failed to lookup property NotificationGroup: no such overload: _+_

CEL returning errors as columns are null type

              results.rows.map(r, {
                'name': r.namespace + '/' + r.name,
                'icon': 'bell',
                'type': 'Notification',
                'health': r.failed == 0 ? 'healthy' : (r.sent > 0 ? 'warning' : 'unhealthy'),
                'properties':[
                  {'name': 'Sent', 'value': r.sent, 'headline': true},
                  {'name': 'Failed', 'value': r.failed, 'headline': true},
                ],
              }).toJSON()