fix: bt fetcher restore

pkg/rest/util/util.go

@@ -21,5 +21,7 @@ func WriteJsonOk(w http.ResponseWriter, v any) {
 func WriteJson(w http.ResponseWriter, code int, v any) {
 	w.Header().Set("Content-Type", "application/json; charset=utf-8")
 	w.WriteHeader(code)
-	json.NewEncoder(w).Encode(v)
+	if v != nil {
+		json.NewEncoder(w).Encode(v)
+	}
 }