Makes fid optionally null so that we can delete the fid later

Summary:
Whenever we set farcaster friends when connecting a new farcaster account, we want to clear previous farcaster entries to keep things up to date.
This means that fid should be optionally null so we can clear without deleting other auxUserStore fields.

Test Plan: dispatched setFarcasterFriendsFIDActionType with null and string fids and checked sqlite database that everything was stored properly

Reviewers: ashoat, ginsu, atul

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11671