[web] [fix] [ENG-977] fix empty chat screen on load

Summary:
fixes: https://linear.app/comm/issue/ENG-977/on-initial-load-a-conversation-should-be-selected

before:

after:

Test Plan: clear application cookies (both user and anonymous) in the browser dev tools. Adding ashoat since we paired on it.

Reviewers: atul, ashoat, palys-swm, def-au1t

Reviewed By: ashoat

Subscribers: Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4040