Request: allow null as user in authentication callbacks

[JuliusNmn]

npm-WebDAV-Server/src/user/v2/authentication/HTTPBasicAuthentication.ts

Line 21 in 5f23762

const onError = (error : Error) =>

Current implementation of callbacks forces you to pass a "default User" on error, which is hacky. The callbacks should allow passing null: webdav.IUser | null.