fix: error when header is undefined

Signed-off-by: seven <zilisheng1996@gmail.com>

Author: Blankll

package-lock.json

@@ -1,6 +1,6 @@
 {
   "name": "@geek-fun/serverless-adapter",
-  "version": "0.1.0",
+  "version": "0.1.1",
   "description": "Adapter for web frame work express, koa, springboot to run in serverless function as backend of apigateway cross multi cloud provider like aliyun, huawei",
   "homepage": "https://www.geekfun.club/",
   "main": "dist/src/index.js",

package.json

@@ -37,7 +37,7 @@ const requestHeaders = (event: Event) => {
   //   }, initialHeader);
   // }
 
-  return Object.keys(event.headers).reduce((headers, key) => {
+  return Object.keys(event.headers ?? {}).reduce((headers, key) => {
     headers[key.toLowerCase()] = event.headers[key];
     return headers;
   }, initialHeader);