DefaultAdapter interface improvements

[yavulan]

Adapter type

Custom adapter

Environment

No impact

Reproduction URL

https://github.com/nextauthjs/next-auth/blob/main/packages/next-auth/src/adapters.ts#L79

Describe the issue

Thank you for this project!
I was implementing a custom adapter and I want to share my thoughts about the DefaultAdapter interface:

• getUserByAccount and unlinkAccount both have a parameter named providerAccountId, but according to the interface it's an object with two fields. Confusing.
• updateUser payload is Partial<AdapterUser>, but how to update without the required user id.
• unlinkAccount doesn't have null in the type union of returned Promise (and does have undefined), but all other similar methods do have null.
• It is unclear why some methods have Promise<void> in return union.
• It is unclear whether the library performs any checks strictly against null and undefined when both appear in the return Promise type union, and which one expected to be returned in what situation.

How to reproduce

Not eligible

Expected behavior

Someone with strong knowledge of how this library works might go through this, filter out relevant suggestions, and safely improve the interface or/and documentation.