Fix generated code problems with nullable primitive return values

[kring]

As a test, I added the following to Runtime/Helpers.cs:

public static bool? TestThing()
{
    return null;
}

And the following to Runtime/ConfigureReinterop.cs:

Helpers.TestThing();

Reinterop generated invalid code for this. After this PR, the code is generated correctly. We previously handled nullable blittable structs, but not nullable primitives.

[j9liu]

This works well on my upgrade-feature-metadata branch so I'm going to merge. Thanks @kring !