dillon1000/react

[Flight] Gated test for dropped transport of undefined object values (#26478)

## Summary

With https://github.com/facebook/react/pull/26349 we now serialize
`undefined`. However, deserializing it on the client is currently
indistinguishable from the value missing entirely due to how
`JSON.parse` treats `undefined` return value of reviver functions.

This leads to inconsistent behavior of the `Object.hasOwn` or `in`
operator (used for narrowing in TypeScript). In TypeScript-speak, `{
prop: T | undefined}` will arrive as `{ prop?: T }`.

## How did you test this change?

- Added test that is expected to fail. Though ideally the implementation
of the component would not care whether it's used on the client or
server.