Fix restore without topic failure (#18387) (#18401)

Co-authored-by: zeripath <art27@cantab.net>

modules/migrations/restore.go

@@ -98,6 +98,9 @@ func (r *RepositoryRestorer) GetTopics() ([]string, error) {
 
 	bs, err := ioutil.ReadFile(p)
 	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, nil
+		}
 		return nil, err
 	}