[lib] Don't show Farcaster prompt for users after they unlink Farcaster

Summary: If the user unlinks their Farcaster identity, we should set it to NO_FID_METADATA so they don't get prompted again.

Test Plan: I looked through the code for places that call useCurrentUserFID to see where undefined/null is differentiated. I saw that ConnectFarcasterAlertHandler calls shouldSkipConnectFarcasterAlert, which differentiates the two. I confirmed that setting NO_FID_METADATA is better, so that we return null and so that ConnectFarcasterAlertHandler doesn't prompt the user to connect their Farcaster account in this case

Reviewers: will

Reviewed By: will

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13735