Try fix tsc error on assigning string

Author: jesstelford

packages/react-html/src/server/tests/stream.test.tsx

@@ -15,7 +15,7 @@ describe('stream', () => {
 
 function streamResponse(stream: NodeJS.ReadableStream) {
   return new Promise<string>((resolve) => {
-    let response: string;
+    let response = '';
 
     stream.on('data', (data) => (response += data));
     stream.on('end', () => resolve(response));

packages/react-server/src/render/tests/render.test.tsx

@@ -248,7 +248,7 @@ describe('createRender', () => {
 
 function readStream(stream: NodeJS.ReadableStream) {
   return new Promise<string>((resolve) => {
-    let response: string;
+    let response = '';
 
     stream.on('data', (data) => (response += data));
     stream.on('end', () => resolve(response));