Fix corner cases in AVM2 class singletons

[kmeisthax]

Our AVM2 implementation currently contains a failsafe for class coercion intended to support writing self-referencing static properties (singletons) in class initializers. However, it does not account for AVM2 properties being nullable, and will spuriously error with Attempted to perform (private) resolution of nonexistent type (type) when coercing null or undefined.

This adds a check for those two values and a test to verify them. As far as I'm aware, we will not need similar failsafes for other primitive values as our AS3 implementations of such do not have self-referencing static properties.

The code pattern this is intended to support is:

class Foo {
    private static var INSTANCE: Foo = null;
}

[adrian17]

This layer of hacks keeps getting bigger ;D