[PORT] Add unit test for CancelAllDialogs and fix eventName and eventValue null

> Port this change from botbuilder-dotnet/master branch:
https://github.com/microsoft/botbuilder-dotnet/pull/3536

#3534 
Fixes CancelAllDialogs.eventName and eventValue is valid to be null


# Changed projects
* Microsoft.Bot.Builder.Dialogs.Adaptive
* Microsoft.Bot.Builder.Dialogs
* Microsoft.Bot.Builder.Dialogs.Adaptive.Tests

[R9,adaptive,bug,test]

