Uncaught (in promise) Error: GraphQL error: Cannot read property 'id' of null

Steps to reproduce:

1. Login as a new user (hasn't entered their email yet)
2. Select a trip from the home page
3. Add trip to cart
4. Go to cart
5. Click Book All

It will explode with: Uncaught (in promise) Error: GraphQL error: Cannot read property 'id' of null.

Looking at the error more closely reveals that we are caching the guest user somewhere. The error comes back from

```
// src/datasources/user.js:35
const userId = this.context.user.id;
if (!userId) return;
```

This means the user context is null

从新用户的复现步骤开始，检查 src/datasources/user.js 第 35 行附近访问 this.context.user.id 的位置。跟踪访客用户在 Book All 期间如何被缓存，然后验证对于没有电子邮件的用户，该流程不再引发报告中的 GraphQL 错误。

由索引模型根据 Issue 内容生成。