[BUG] [typescript-fetch] Compilation failure in typescript build

Created by: IodizedGabe

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
Have you validated the input using an OpenAPI validator (example)?
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Description

The typescript-fetch client fails to compile.

Compilation failure:

    src/ext/runtime.ts:152:9 - error TS2322: Type '((input: RequestInfo, init?: RequestInit | undefined) => Promise<Response>) | undefined' is not assignable to type '(input: RequestInfo, init?: RequestInit | undefined) => Promise<Response>'.
      Type 'undefined' is not assignable to type '(input: RequestInfo, init?: RequestInit | undefined) => Promise<Response>'.

    152         return this.configuration.fetchApi;
                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

openapi-generator version

Version: v5.2.1 Regression: Yes

Generation Details

CLI used:

docker run -v "${PWD}:/local" openapitools/openapi-generator-cli:v5.2.1 generate -i /local/<specfile> -g typescript-fetch

Steps to reproduce

See above

Related issues/PRs

This was introduced in: https://github.com/OpenAPITools/openapi-generator/commit/c05ec99182c80993a218a841853ba6412e6a6c16

This causes an error because this config value may be undefined: https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache#L122

Suggest a fix

The fallback as seen here: https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache#L122

Could be added to the getter.