signinPopup in AuthContextProps should return Promise<User | null>

[benh-dug]

As a follow-up to #820 (comment) I believe that the 'signinPopup' function should return a possibly null value.

I ran into a similar issue to the linked comment where the promise actually resolved to null (When I closed the popup) and I didn't have any typing warning me that could happen.

I'd be happy to raise a PR if this is expected behaviour.