[web] Replace {||} with {} in identity-structs.cjs.flow

Summary:
Following the Flow upgrade, I was experimenting with the various codemods in flow-upgrade. I gave removeExplicitlyExactObjectTypeSyntax a try, since we have exact_by_default=true set in our .flowconfigs, and as such {} is an alias to {||}.

The codemod found a couple locations in this file. For the sake of consistency, I replaced them with {}, like we do elsewhere in our codebase.

Depends on D10087

Test Plan: Flow

Reviewers: varun, bartek

Reviewed By: bartek

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10088