Make MaxOverSubscriptionRatio optional

since this field is optional we have to provide a possibility to
leave it empty

Change-Id: Ida453a1b09b0c493f823ecf50c343f4230b7697a
Related-PROD: PROD-33588
diff --git a/openstack/blockstorage/extensions/schedulerstats/results.go b/openstack/blockstorage/extensions/schedulerstats/results.go
index 95eaaed..0dc47ec 100644
--- a/openstack/blockstorage/extensions/schedulerstats/results.go
+++ b/openstack/blockstorage/extensions/schedulerstats/results.go
@@ -87,6 +87,8 @@
 		r.MaxOverSubscriptionRatio = t
 	case float64:
 		r.MaxOverSubscriptionRatio = strconv.FormatFloat(t, 'f', -1, 64)
+	case nil:
+		r.MaxOverSubscriptionRatio = ""
 	default:
 		return fmt.Errorf("MaxOverSubscriptionRatio has unexpected type: %T", t)
 	}