[Flight] Add support for returning undefined from render (#26349)

## Summary
Adds support for returning `undefined` from Server Components.
Also fixes a bug where rendering an empty fragment would throw the same
error as returning undefined.
## How did you test this change?
- [x] test failed with same error message I got when returning undefined
from Server Components in a Next.js app
- [x] test passes after adding encoding for `undefined`

Author: eps1lon

packages/react-client/src/ReactFlightClient.js

@@ -556,6 +556,11 @@ export function parseModelString(
 throwchunk.reason;
 }
 }
+case'u': {
+// matches "$undefined"
+// Special encoding for `undefined` which can't be serialized as JSON otherwise.
+ return undefined;
+}
 default: {
 // We assume that anything else is a reference ID.
 constid=parseInt(value.substring(1),16);

packages/react-client/src/__tests__/ReactFlight-test.js

@@ -197,6 +197,38 @@ describe('ReactFlight', () => {
 expect(ReactNoop).toMatchRenderedOutput(<span>ABC</span>);
 });
 
+it('can render undefined',async()=>{
+functionUndefined(){
+returnundefined;
+}
+
+constmodel=<Undefined/>;
+
+consttransport=ReactNoopFlightServer.render(model);
+
+awaitact(async()=>{
+ReactNoop.render(awaitReactNoopFlightClient.read(transport));
+});
+
+expect(ReactNoop).toMatchRenderedOutput(null);
+});
+
+it('can render an empty fragment',async()=>{
+functionEmpty(){
+return<React.Fragment/>;
+}
+
+constmodel=<Empty/>;
+
+consttransport=ReactNoopFlightServer.render(model);
+
+awaitact(async()=>{
+ReactNoop.render(awaitReactNoopFlightClient.read(transport));
+});
+
+expect(ReactNoop).toMatchRenderedOutput(null);
+});
+
 it('can render a lazy component as a shared component on the server',async()=>{
 functionSharedComponent({text}){
 return(

packages/react-server/src/ReactFlightServer.js

@@ -117,6 +117,7 @@ export type ReactClientValue =
 |number
 |symbol
 |null
+|void
 |Iterable<ReactClientValue>
 |Array<ReactClientValue>
 |ReactClientObject
@@ -546,6 +547,10 @@ function serializeProviderReference(name: string): string {
 return'$P'+name;
 }
 
+functionserializeUndefined(): string{
+return'$undefined';
+}
+
 functionserializeClientReference(
 request: Request,
 parent:
@@ -1134,14 +1139,14 @@ export function resolveModelToJSON(
 returnescapeStringValue(value);
 }
 
-if(
-typeofvalue==='boolean'||
-typeofvalue==='number'||
-typeofvalue==='undefined'
-){
+if(typeofvalue==='boolean'||typeofvalue==='number'){
 returnvalue;
 }
 
+if(typeofvalue==='undefined'){
+returnserializeUndefined();
+}
+
 if(typeofvalue==='function'){
 if(isClientReference(value)){
 returnserializeClientReference(request,parent,key,(value: any));