[lib] Don't insert username into UserStore when UserInfo missing

Summary:
This addresses ENG-9373.

What's happening here:

1. When findUserIdentities starts, there is a UserInfo for this user in the store
2. By the time findUserIdentitiesActionTypes.success is reduced, the UserInfo is empty, so we end up inserting just the username into the store
3. As a result, we end up passing a UserInfo without an ID to C++, which causes a crash

Test Plan:
I tested the following 5 times and confirmed that it didn't cause a crash:

1. Create a test account on iOS Simulator
2. Create a test account on Android
3. Send a friend request from Android to iOS (necessary to create a relationship)
4. Delete account on Android

Reviewers: varun, will

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13494