node/meta: do not panic if garbage/graveyard bucket is missing

They should exist normally, but we also use this code in neofs-lens that can
work with any metabase in any state. Technically, it is also correct: missing
graveyard iteration may be considered no-op.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

pkg/local_object_storage/metabase/graveyard.go

@@ -188,6 +188,10 @@ func (db *DB) iterateDeletedObj(tx *bbolt.Tx, h kvHandler, offset *oid.Address)
 		panic(fmt.Sprintf("metabase: unknown iteration object hadler: %T", t))
 	}
 
+	if bkt == nil {
+		return nil
+	}
+
 	c := bkt.Cursor()
 	var k, v []byte