Don't create AnyOf instances with null values

[ob-stripe]

r? @brandur-stripe
cc @stripe/api-libraries

Modifies the implicit operators used to create AnyOf<> instances from value types to return null instead of creating a new AnyOf instance that contains null when the value is itself null,

Previous behavior:

AnyOf<int, string> anyOf = null;
anyOf = (string)null;
Assert.NotNull(anyOf);
// anyOf is now a (non-null) AnyOf<int, string> instance with `Value` set to `null`

New behavior:

AnyOf<int, string> anyOf = null;
anyOf = (string)null;
Assert.Null(anyOf);
// anyOf is still null

I've also changed ToString(), as previously it would crash if Value was null.

[ob-stripe]

Unfortunately, because this is a generic class, I don't think it's possible to write a helper method to do this in a single place.

[brandur-stripe]

OOC, was there some particular impetus for this, or did you just notice recently and decide to try to make it more correct?

Regardless, seems like a good improvement — LGTM.

The PR description examples were super helpful BTW — thanks!

[ob-stripe]

OOC, was there some particular impetus for this, or did you just notice recently and decide to try to make it more correct?

A user ran into this when writing tests for their Stripe integration.

The previous behavior was not strictly a bug since the library would omit the parameter in the request in both cases (whether the AnyOf property is null or is non-null but contains a null value), but they were surprised that they were getting back a non-null value after explicitly assigning a null value.

[ob-stripe]

Released as 27.9.1.