[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`

DiffTrain build for [d1ad984](d1ad984)

compiled/facebook-www/REVISION

@@ -1 +1 @@
-3706edb81c84ea01e68bfb477537ba9908e1bd70
+d1ad984db1591b131d16739a24dee4ba44886a09

compiled/facebook-www/ReactFlightDOMRelayClient-dev.classic.js

@@ -555,6 +555,12 @@ function parseModelString(response, parentObject, key, value) {
         }
       }
 
+      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.
         var _id3 = parseInt(value.substring(1), 16);

compiled/facebook-www/ReactFlightDOMRelayClient-dev.modern.js

@@ -555,6 +555,12 @@ function parseModelString(response, parentObject, key, value) {
         }
       }
 
+      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.
         var _id3 = parseInt(value.substring(1), 16);

compiled/facebook-www/ReactFlightDOMRelayClient-prod.classic.js

@@ -282,6 +282,8 @@ function parseModelString(response, parentObject, key, value) {
           default:
             throw parentObject.reason;
         }
+      case "u":
+        return;
       default:
         value = parseInt(value.substring(1), 16);
         response = getChunk(response, value);

compiled/facebook-www/ReactFlightDOMRelayClient-prod.modern.js

@@ -282,6 +282,8 @@ function parseModelString(response, parentObject, key, value) {
           default:
             throw parentObject.reason;
         }
+      case "u":
+        return;
       default:
         value = parseInt(value.substring(1), 16);
         response = getChunk(response, value);

compiled/facebook-www/ReactFlightDOMRelayServer-dev.classic.js

@@ -1637,6 +1637,10 @@ function serializeProviderReference(name) {
   return "$P" + name;
 }
 
+function serializeUndefined() {
+  return "$undefined";
+}
+
 function serializeClientReference(request, parent, key, clientReference) {
   var clientReferenceKey = getClientReferenceKey(clientReference);
   var writtenClientReferences = request.writtenClientReferences;
@@ -2235,14 +2239,14 @@ function resolveModelToJSON(request, parent, key, value) {
     return escapeStringValue(value);
   }
 
-  if (
-    typeof value === "boolean" ||
-    typeof value === "number" ||
-    typeof value === "undefined"
-  ) {
+  if (typeof value === "boolean" || typeof value === "number") {
     return value;
   }
 
+  if (typeof value === "undefined") {
+    return serializeUndefined();
+  }
+
   if (typeof value === "function") {
     if (isClientReference(value)) {
       return serializeClientReference(request, parent, key, value);

compiled/facebook-www/ReactFlightDOMRelayServer-dev.modern.js

@@ -1637,6 +1637,10 @@ function serializeProviderReference(name) {
   return "$P" + name;
 }
 
+function serializeUndefined() {
+  return "$undefined";
+}
+
 function serializeClientReference(request, parent, key, clientReference) {
   var clientReferenceKey = getClientReferenceKey(clientReference);
   var writtenClientReferences = request.writtenClientReferences;
@@ -2235,14 +2239,14 @@ function resolveModelToJSON(request, parent, key, value) {
     return escapeStringValue(value);
   }
 
-  if (
-    typeof value === "boolean" ||
-    typeof value === "number" ||
-    typeof value === "undefined"
-  ) {
+  if (typeof value === "boolean" || typeof value === "number") {
     return value;
   }
 
+  if (typeof value === "undefined") {
+    return serializeUndefined();
+  }
+
   if (typeof value === "function") {
     if (isClientReference(value)) {
       return serializeClientReference(request, parent, key, value);

compiled/facebook-www/ReactFlightDOMRelayServer-prod.classic.js

@@ -937,12 +937,8 @@ function resolveModelToJSON(request, parent, key, value) {
   }
   if ("string" === typeof value)
     return (request = "$" === value[0] ? "$" + value : value), request;
-  if (
-    "boolean" === typeof value ||
-    "number" === typeof value ||
-    "undefined" === typeof value
-  )
-    return value;
+  if ("boolean" === typeof value || "number" === typeof value) return value;
+  if ("undefined" === typeof value) return "$undefined";
   if ("function" === typeof value) {
     if (value instanceof JSResourceReferenceImpl)
       return serializeClientReference(request, parent, key, value);

compiled/facebook-www/ReactFlightDOMRelayServer-prod.modern.js

@@ -937,12 +937,8 @@ function resolveModelToJSON(request, parent, key, value) {
   }
   if ("string" === typeof value)
     return (request = "$" === value[0] ? "$" + value : value), request;
-  if (
-    "boolean" === typeof value ||
-    "number" === typeof value ||
-    "undefined" === typeof value
-  )
-    return value;
+  if ("boolean" === typeof value || "number" === typeof value) return value;
+  if ("undefined" === typeof value) return "$undefined";
   if ("function" === typeof value) {
     if (value instanceof JSResourceReferenceImpl)
       return serializeClientReference(request, parent, key, value);