[web/native] Return null instead of throw new error when incomplete auth metadata

Summary:
This returns null when the auth metadata returned is incomplete (contains null values).

Context: https://linear.app/comm/issue/ENG-7083/getting-auth-metadata-is-incomplete-error-when-testing-on-web

Test Plan: no longer receive uncaught promise error on web

Reviewers: varun, ashoat

Reviewed By: varun, ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11198