Adds check to consider the required flag on multipart forms in the C# Code generator

[simonnieder]

The problem is that for multipart forms the required fields are ignored. This causes the client to throw an ArgumentNullException even if the parameter is NOT required.

For

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

we would expect

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

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

However, we get

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

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

[AroglDarthu]

I think this would also solve issue #4174 and #4518 right?
@RicoSuter please accept the PR!