[BUG][typescript-inversify] Type mismatch between service model and HttpClient requirement

Created by: siada

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
Have you validated the input using an OpenAPI validator (example)?
What's the version of OpenAPI Generator used?
Have you search for related issues/PRs?
What's the actual output vs expected output?
[Optional] Bounty to sponsor the fix (example)

Description

When using typescript in strict mode, the code won't compile if your spec has an optional requestBody because the HttpClient defines body as {} which means 'any except null or undefined'

openapi-generator version

4.1.0

Steps to reproduce

Generated service method:

    public apiAccountLoginPost(loginViewModel?: LoginViewModel, observe?: 'body', headers?: Headers): Promise<LoginViewModelResult>;
    public apiAccountLoginPost(loginViewModel?: LoginViewModel, observe?: 'response', headers?: Headers): Promise<HttpResponse<LoginViewModelResult>>;
    public apiAccountLoginPost(loginViewModel?: LoginViewModel, observe: any = 'body', headers: Headers = {}): Promise<any> {

HttpClient:

    post(url: string, body: {}|FormData, headers?: Headers): Observable<HttpResponse> {
        return this.performNetworkCall(url, "POST", this.getJsonBody(body), this.addJsonHeaders(headers));
    }

Error:

Related issues/PRs

#3607

Suggest a fix

I believe changing the {} to any in the HttpClient will resolve this, however I don't know enough about typescript to understand the impact this may have

I'm happy to organise the PR for this if you agree with this change