fix(fetch): prevent TypeError when config.env is undefined (#7155)

lib/adapters/fetch.js

@@ -262,7 +262,7 @@ const factory = (env) => {
 const seedCache = new Map();
 
 export const getFetch = (config) => {
-  let env = config ? config.env : {};
+  let env = (config && config.env) || {};
   const {fetch, Request, Response} = env;
   const seeds = [
     Request, Response, fetch