[lib] Don't remove avatar when fetching username

Summary: When findUserIdentitiesActionTypes.success is dispatched, an entry for this id may already exist in the store - we may have gotten the user infos from auth keyserver already. We don't want to forget other values of this user info. Namely - avatar and relationshipStatus.

Test Plan: Dispatched findUserIdentitiesActionTypes.success without avatar for an entry that existed in the store and had an avatar. Checked that the updated entry still had the avatar.

Reviewers: tomek, kamil

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12318