Improvements to Adapter: unlinkAccount and updateUser

[bjoernwuest]

Goals

Simplify / streamline Adapter API.

Non-Goals

Introduce new functionality

Background

In current Adapter interface, following functions are defined which deviate from all other function specifications:

• unlinkAccount: Promise<undefined | AdapterAccount> ... as only function, this returns "undefined" whereas other functions return null.
• updateUser: Promise<AdapterUser> ... this function must return a user, even no user to update was found!

Proposal

Update functions to following syntax:

• unlinkAccount(...): Promise<null | AdapterAccount>
• updateUser(...): Promise<null | AdapterUser>