[lib] Remove !threadInfos check from BaseAutoJoinCommunityHandler

Summary:
Ginsu added this check in this revision of the diff that first introduced this code.

My understanding is that threadInfos should always be set, and that's how we treat it elsewhere in the codebase. I think it's safe to remove this check.

Test Plan: Flow, and also checking with Ginsu about why he added the check

Reviewers: varun

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13626