[BUG] [csharp-netcore] Don't check integers if they are null

Created by: tndata

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?
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Description

The generation works fine, but I get warnings by the compiler in Visual Studio due to code like this:

   public Org.OpenAPITools.Client.ApiResponse< CustomerExternalServicesModelCollection > GetCustomerExternalServicesWithHttpInfo (int customerId)
    {
        // verify the required parameter 'customerId' is set
        if (customerId == null)
            throw new Org.OpenAPITools.Client.ApiException(400, "Missing required parameter 'customerId' when calling CustomerExternalServicesApi->GetCustomerExternalServices");

.....

openapi-generator version

4.2.1

OpenAPI declaration file content or url

https://leksaker.starweb.se/api/v2/openapi.json

Command line used for generation

npx openapi-generator generate -i https://leksaker.starweb.se/api/v2/openapi.json -g csharp-netcore -o /temp/swagger

Suggest a fix

Don't include null-checks against integer input parameters.