Fix nil pointer panic when exec some gitea cli command (#28791)

panic:

![image](https://github.com/go-gitea/gitea/assets/18380374/7fcde2ad-1d42-4b60-b120-3b60a8926e8e)

After:

![image](https://github.com/go-gitea/gitea/assets/18380374/49d9f0ca-e590-4a35-8ca2-1317d1b7c939)

modules/private/mail.go

@@ -30,5 +30,8 @@ func SendEmail(ctx context.Context, subject, message string, to []string) (strin
 	})
 
 	resp, extra := requestJSONResp(req, &responseText{})
+	if resp == nil {
+		return "", extra
+	}
 	return resp.Text, extra
 }