fix(ext/node): ServerResponse getHeader() return undefined (#21525)

Matches Node's return type.

Next.js check for `if (header === undefined)`:

https://github.com/vercel/next.js/blob/e02fe314dcd0ae614c65b505c6daafbdeebb920e/packages/next/src/server/base-http/node.ts#L93

1 files changed, 1 insertions, 1 deletions

diff --git a/ext/node/polyfills/http.ts b/ext/node/polyfills/http.ts
index 27fc577c0..d45854162 100644
--- a/ext/node/polyfills/http.ts
+++ b/ext/node/polyfills/http.ts
@@ -1394,7 +1394,7 @@ export class ServerResponse extends NodeWritable {
   }
 
   getHeader(name: string) {
-    return this.#headers.get(name);
+    return this.#headers.get(name) ?? undefined;
   }
   removeHeader(name: string) {
     return this.#headers.delete(name);