fix typescript-fetch typos

It looks like there's a couple typos that cause compilation to fail. I implemented the first two changes suggested in swagger-api#11870 as that is what allowed me to build in typescript 4.9.5.

Author: chrisdrifte

modules/swagger-codegen/src/main/resources/typescript-fetch/api.mustache

@@ -44,7 +44,7 @@ export interface FetchArgs {
  * @class BaseAPI
  */
 export class BaseAPI {
-    protected configuration: Configuration;
+    protected configuration?: Configuration;
 
     constructor(configuration?: Configuration, protected basePath: string = BASE_PATH, protected fetch: FetchAPI = portableFetch) {
         if (configuration) {
@@ -61,7 +61,7 @@ export class BaseAPI {
  * @extends {Error}
  */
 export class RequiredError extends Error {
-    name: "RequiredError"
+    name = "RequiredError"
     constructor(public field: string, msg?: string) {
         super(msg);
     }