[kotlin][client] only use authentication tokens when they are not null

Created by: 4brunu

When using bearer authentication or oauth, the tokens are used even if they are null.

Here is the generated code:

    protected fun updateAuthParams(requestConfig: RequestConfig) {
        if (requestConfig.headers[Authorization].isNullOrEmpty()) {
            requestConfig.headers[Authorization] = "Bearer " + accessToken
        }
    }

The problem is that if accessToken is null, the header "Authentication" will have the value "Bearer null" and the server will return Http Status Code 500 instead of 401.

Instead of the above code, I suggest generating something like this.

    protected fun updateAuthParams(requestConfig: RequestConfig) {
        if (requestConfig.headers[Authorization].isNullOrEmpty()) {
            accessToken?.let { accessToken ->
                requestConfig.headers[Authorization] = "Bearer " + accessToken
            }
        }
    }

Fixes https://github.com/OpenAPITools/openapi-generator/issues/4237

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