[Flight] Fix debug info leaking to outer handler (facebook#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.

Author: sebmarkbage

packages/react-client/src/ReactFlightClient.js

@@ -879,7 +879,7 @@ function initializeDebugChunk(
             waitForReference(
               debugChunk,
               {}, // noop, since we'll have already added an entry to debug info
-              '', // noop
+              'debug', // noop, but we need it to not be empty string since that indicates the root object
               response,
               initializeDebugInfo,
               [''], // path

packages/react-server-dom-webpack/src/__tests__/ReactFlightDOMEdge-test.js

@@ -1915,4 +1915,57 @@ describe('ReactFlightDOMEdge', () => {
       expect(ownerStack).toBeNull();
     }
   });
+
+  it('can pass an async import that resolves later as a prop to a null component', async () => {
+    let resolveClientComponentChunk;
+    const client = clientExports(
+      {
+        foo: 'bar',
+      },
+      '42',
+      '/test.js',
+      new Promise(resolve => (resolveClientComponentChunk = resolve)),
+    );
+
+    function ServerComponent(props) {
+      return null;
+    }
+
+    function App() {
+      return (
+        <div>
+          <ServerComponent client={client} />
+        </div>
+      );
+    }
+
+    const stream = await serverAct(() =>
+      passThrough(
+        ReactServerDOMServer.renderToReadableStream(<App />, webpackMap),
+      ),
+    );
+
+    // Parsing the root blocks because the module hasn't loaded yet
+    const response = ReactServerDOMClient.createFromReadableStream(stream, {
+      serverConsumerManifest: {
+        moduleMap: null,
+        moduleLoading: null,
+      },
+    });
+
+    function ClientRoot() {
+      return use(response);
+    }
+
+    // Initialize to be blocked.
+    response.then(() => {});
+    // Unblock.
+    resolveClientComponentChunk();
+
+    const ssrStream = await serverAct(() =>
+      ReactDOMServer.renderToReadableStream(<ClientRoot />),
+    );
+    const result = await readResult(ssrStream);
+    expect(result).toEqual('<div></div>');
+  });
 });