Fix nullable scenarios when xaml asks for type and fix vtable for event handlers

[manodasanW]

• PropertyChangedEventHandler and NotifyCollectionChangedEventHandler are both custom type mapped delegates and were missing vtable entries. This adds vtable entries for them along with tests.
• WinUI has scenarios where it asks to marshal a native type name to a managed type. We are seeing scenarios where WinUI is asking for nullable types and we aren't able to return the actual type given we avoid MakeGenericType on AOT. Typically, this would be fine if this is being round trip to the native side given we are able to return a fake type, but there are scenarios where WinUI is doing an equality check on the type we return to find the entry for it on its managed lookup table. That is failing when we return our fake type. Addressing that for built-in types by adding an explicit check to handle them.

Fixes #1678