Weakly-typed actors should support polymorphic response and null response

[RemcoBlok]

Expected Behavior

1. In .NET 7.0 or later an operation on an actor invoked using a weakly-typed actor client should support a polymorphic response. The response json should include a type discriminator property to allow for polymorphic deserialization by the actor client. The weakly-typed actor client must polymorphically deserialize the response when invoking InvokeMethodAsync<ResponseBase> on the weakly-typed actor client. This should yield a DerivedResponse instance. Note that invoking InvokeMethodAsync<DerivedResponse> is not polymorphic deserialization.
2. An operation on an actor invoked using a weakly-typed actor client should support a null response.

Actual Behavior

1. The response json does not include a type discriminator. The deserialized response is an instance of the base response class, not the derived response class.
2. A null reference exception is thrown by the ActorManager.DispatchWithoutRemotingAsync method.

Steps to Reproduce the Problem

1. Declare an operation on an actor interface whose declared return type is a base class called ResponseBase. Implement the operation on an actor. The implementation returns an instance of a derived class called DerivedResponse. Invoke the operation using a weakly-typed actor client using the InvokeMethodAsync<ResponseBase> method (not using InvokeMethodAsync<DerivedResponse>).
2. Return null from an actor operation invoked using a weakly-typed actor client.

Release Note

1. RELEASE NOTE: Weakly-typed actor supports polymorphic response in .NET 7 or later.
2. RELEASE NOTE: Weakly-typed actor supports null response.