[bug]发送webhook请求时，screenshot字段值为空

[KexinCC]

• 系统环境 / System Environment：

• BetterGI版本号 / BetterGI Version：0.53.0

• 问题描述 / Description of the issue：发送webhook请求时，screenshot字段值为空

• 23:37:22.040 [http-nio-0.0.0.0-8080-exec-4] INFO o.x.d.controller.WebhookController - 📥 收到 Webhook 请求
  23:37:22.040 [http-nio-0.0.0.0-8080-exec-4] DEBUG o.x.d.controller.WebhookController - 📄 Raw Request Body: {"send_to":"","event":"group.end","result":0,"timestamp":"2025-12-01T23:37:22.0398671+08:00","screenshot":null,"message":"\u914D\u7F6E\u7EC4\u5FAE\u5149\u89D2\u83CC\u7ED3\u675F","data":null}
  23:37:22.040 [http-nio-0.0.0.0-8080-exec-4] INFO o.x.d.controller.WebhookController - ✅ Webhook 处理成功

• 复现步骤 / Reproduction steps：

[zaodonganqi]

全局通知设置里打开截图了吗？