fix: use undefined instead of null for empty request context in nextjs adapter

Co-authored-by: hotlong <50353452+hotlong@users.noreply.github.com>

Author: Copilot

packages/adapters/nextjs/src/index.ts

@@ -199,7 +199,7 @@ export interface ServerActionResult<T = any> {
  */
 export function createServerActions(options: NextAdapterOptions) {
   const dispatcher = new HttpDispatcher(options.kernel);
-  const emptyContext = { request: null };
+  const emptyContext = { request: undefined };
 
   return {
     /**