fix: marshal APIResponse error

types.go

@@ -51,6 +51,9 @@ type APIResponse struct {
 // String dumps JSON without tailing \n
 func (resp *APIResponse) String() string {
 	sb := strings.Builder{}
+	if len(resp.Data) == 0 {
+		resp.Data = []byte(`null`)
+	}
 	err := json.NewEncoder(&sb).Encode(resp)
 	if err != nil {
 		panic(errors.Wrap(err, "unexpected"))