[lib] Fix auth with non authoritative keyserver error

Summary:
This fixes a problem described in this comment: https://linear.app/comm/issue/ENG-7312/terms-of-service-modal-popping-up-unexpectedly#comment-64686c0a
We can auth with a different keyserver than the authoritative one, so responses[authoritativeKeyserverID()] may not exist

Test Plan: Reading the code - since responses[authoritativeKeyserverID()] may not exist, we have to do this. We also do this for currentUserInfo a couple of lines higher

Reviewers: tomek, michal, kamil

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11365