Update snapshot parent filter property to always exist

The parent property may be empty but always exists.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

snapshots/storage/bolt.go

@@ -633,9 +633,7 @@ func adaptSnapshot(info snapshots.Info) filters.Adaptor {
 		case "name":
 			return info.Name, true
 		case "parent":
-			if info.Parent != "" {
-				return info.Parent, true
-			}
+			return info.Parent, true
 		case "labels":
 			if len(info.Labels) == 0 {
 				return "", false