[PORT] Add unit test for CancelAllDialogs and fix eventName and eventValue null #857
Description
Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#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]