[lib] Discard keyserver usernames for processServerRequestsActionType

Summary: issue: ENG-8365

Test Plan: Tested that if processServerRequestsActionType is dispatched with some userInfos in stateChanges and those users are not in the store, the username is set to null

Reviewers: tomek, kamil

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12513