Fix CS8625 `Cannot convert null literal to non-nullable reference type` warning in QuestionAnsweringDialog.cs

[HugoRamosEs]

The problem arises because the variable "object options" is assigned a null value, indicating that it cannot convert a null literal to a non-nullable reference type. We cannot mark it as nullable because it would disrupt the implementation of the public interface Microsoft.Bot.Builder.Dialogs.Dialog.