msgstore_fs: fix ListTargets error on missing log dir

Initially, before connecting to the network, the log dir will be
empty. Return an empty list of chat history targets in this case.

msgstore_fs.go

@@ -480,7 +480,9 @@ func (ms *fsMessageStore) ListTargets(network *network, start, end time.Time, li
 	end = end.In(time.Local)
 	rootPath := filepath.Join(ms.root, escapeFilename(network.GetName()))
 	root, err := os.Open(rootPath)
-	if err != nil {
+	if os.IsNotExist(err) {
+		return nil, nil
+	} else if err != nil {
 		return nil, err
 	}