no-null-in-aliases

When you read User, you expect a user - not maybe-a-user. Include null explicitly at usage sites instead of hiding it in type aliases.

Then the optional chain ?. is needed. But readers can't tell without checking the definition.

This rule applies to the top level of type aliases. Nullable properties inside objects are fine:

在定义类型别名时使用。当类型定义中出现 null/undefined 时使用。当类型令人困惑时使用。 来源：marius-townhouse/effective-typescript-skills。