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:

在定義類型別名時使用。當類型定義中出現 null/undefined 時使用。當類型令人困惑時使用。 來源：marius-townhouse/effective-typescript-skills。