Remove null-checks for non-required properties (csharp code generation)

[josch83]

Same as described in #5023:

Given:

            "multipart/form-data": {
              "schema": {
                "required": [
                  "file"
                ],
                "type": "object",
                "properties": {
                  "file": {
                    "type": "string",
                    "description": "",
                    "format": "binary"
                  },
                  "fileDescription": {
                    "type": "string",
                    "description": ""
                  }
                }
              }, ...

Instead of:

                    if (file == null)
                        throw new System.ArgumentNullException("file");
                    else
                    {
                     ...
                    }

                    if (fileDescription != null)
                    {
                     ...
                    }

It generates:

                    if (file == null)
                        throw new System.ArgumentNullException("file");
                    else
                    {
                     ...
                    }

                    if (fileDescription == null)
                        throw new System.ArgumentNullException("fileDescription");
                    else
                    {
                     ...
                    }

This PR resolves the issue with a different approach (tested and used in our application)