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

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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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.

Start with modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache at the referenced getter and compare it with the fallback already used at line 122. Generate a typescript-fetch client with the v5.2.1 Docker command, run its TypeScript build, and confirm compilation succeeds without the TS2322 error.