[DPG] A property defined in TypeSpec as "MyExtensibleEnum | null" should be generated as nullable

[joseharriaga]

Using version 20230714.1.

We're seeing this in the OpenAI library. Consider the following example in TypeSpec:

enum MyExtensibleEnum{
  foo: "foo",
  bar: "bar",
}

model MyModel {
  myProperty: MyExtensibleEnum | null;
}

The code that is being generated for this model looks like this:

public partial class MyModel
{
    public MyExtensibleEnum MyProperty { get; }
}

And its deserialization code looks like this:

if (property.NameEquals("myProperty"u8))
{
    myProperty = new MyExtensibleEnum(property.Value.GetString());
    continue;
}

In other words, MyProperty is not nullable, as opposed to what is specified in the TypeSpec.