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

Summary:
Following the Flow upgrade, I was experimenting with the various [codemods in](https://www.npmjs.com/package/flow-upgrade) `flow-upgrade`. I gave `removeExplicitlyExactObjectTypeSyntax` a try, since we have `exact_by_default=true` set in our `.flowconfig`s, 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

web/protobufs/identity-structs.cjs.flow

@@ -9,8 +9,8 @@ import {
 
 declare export class Empty extends Message {
   serializeBinary(): Uint8Array;
-  toObject(includeInstance?: boolean): {||};
-  static toObject(includeInstance: boolean, msg: Empty): {||};
+  toObject(includeInstance?: boolean): {};
+  static toObject(includeInstance: boolean, msg: Empty): {};
   static serializeBinaryToWriter(message: Empty, writer: BinaryWriter): void;
   static deserializeBinary(bytes: Uint8Array): Empty;
   static deserializeBinaryFromReader(message: Empty, reader: BinaryReader): Empty;