dillon1000/react

[Flight] Fix debug info leaking to outer handler (#34081)

The `waitForReference` call for debug info can trigger inside a
different object's initializingHandler. In that case, we can get
confused by which one is the root object.

We have this special case to detect if the initializing handler's object
is `null` and we have an empty string key, then we should replace the
root object's value with the resolved value.


https://github.com/facebook/react/blob/52612a7cbdd8e1fee9599478247f78725869ebad/packages/react-client/src/ReactFlightClient.js#L1374

However, if the initializing handler actually should have the value
`null` then we might get confused by this and replace it with the
resolved value from a debug object. This fixes it by just using a
non-empty string as the key for the waitForReference on debug value
since we're not going to use it anyway.

It used to be impossible to get into this state since a `null` value at
the root couldn't have any reference inside itself but now the debug
info for a `null` value can have outstanding references.

However, a better fix might be using a placeholder marker object instead
of null or better yet ensuring that we know which root we're
initializing in the debug model.