default to empty string identifier prefix

Author: gnoff

packages/react-server/src/ReactFlightHooks.js

@@ -107,10 +107,7 @@ function useId(): string {
   if (currentRequest === null) {
     throw new Error('useId can only be used while React is rendering');
   }
-  const prefix = currentRequest.identifierPrefix
-    ? currentRequest.identifierPrefix
-    : '';
   const id = currentRequest.identifierCount++;
   // use 'S' for Flight components to distinguish from 'R' and 'r' in Fizz/Client
-  return ':' + prefix + 'S' + id.toString(32) + ':';
+  return ':' + currentRequest.identifierPrefix + 'S' + id.toString(32) + ':';
 }

packages/react-server/src/ReactFlightServer.js

@@ -108,7 +108,7 @@ export type Request = {
   writtenSymbols: Map<Symbol, number>,
   writtenModules: Map<ModuleKey, number>,
   writtenProviders: Map<string, number>,
-  identifierPrefix?: string,
+  identifierPrefix: string,
   identifierCount: number,
   onError: (error: mixed) => void,
   toJSON: (key: string, value: ReactModel) => ReactJSONValue,
@@ -148,7 +148,7 @@ export function createRequest(
     writtenSymbols: new Map(),
     writtenModules: new Map(),
     writtenProviders: new Map(),
-    identifierPrefix,
+    identifierPrefix: identifierPrefix || '',
     identifierCount: 1,
     onError: onError === undefined ? defaultErrorHandler : onError,
     toJSON: function(key: string, value: ReactModel): ReactJSONValue {