[BUG][C#][csharp-netcore] Many warnings detected when building generated code

Created by: 414004738

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?
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What's the actual output vs expected output?
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Description

When building a generated client using the csharp-netcore generator with a targetFramework of netcoreapp2.0, there are many compilation warning detected. When using the test yaml from here: https://github.com/OpenAPITools/openapi-generator/blob/v4.1.0/modules/openapi-generator/src/test/resources/2_0/petstore-with-fake-endpoints-models-for-testing.yaml, 70 warnings are detected.

Most of the warnings are of the form:

PetApi.cs(713, 17): [CS0472] The result of the expression is always 'false' since a value of type 'long' is never equal to 'null' of type 'long?'

Expected behavior is for no warnings to be generated.

openapi-generator version

4.1.0

OpenAPI declaration file content or url

I used the test yaml here: https://github.com/OpenAPITools/openapi-generator/blob/v4.1.0/modules/openapi-generator/src/test/resources/2_0/petstore-with-fake-endpoints-models-for-testing.yaml

Command line used for generation

openapi-generator generate -g csharp-netcore -i ./petstore-with-fake-endpoints-models-for-testing.yaml --additional-properties targetFramework=netcoreapp2.0 -o ./netcoreapp2.0

Steps to reproduce

Run the command line above and build the project. Look at the compiler warnings.