[PORT] Add empty recognizer support for CrosstrainedRecognizer (composer scenario)

[tomlm]

Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#4144

Fixes #

Description

We plan to set default recognizer in composer as Luis+QnA CrosstrainedRecognizer.
And it will fit into different recognizer types by the .lu/.qna content.

Recognizer: "dialogname.lu.qna"

dialogname.lu.qna,dialog will be generated as the following rule:

.lu and .qna both not empty =>

{ 
    "$kind": "Microsoft.CrossTrainedRecognizerSet" 
    recongizers:[ 
        "dialogname.lu", 
        "dialogname.qna” 
   ] 
} 

.lu and .qna both empty =>

{ 
    "$kind": "Microsoft.CrossTrainedRecognizerSet" 
    recongizers:[ 
   ] 
} 

.lu empty =>

{ 
    "$kind": "Microsoft.CrossTrainedRecognizerSet" 
    recongizers:[ 
        "dialogname.qna” 
   ] 
} 

.qna empty =>

{ 
    "$kind": "Microsoft.CrossTrainedRecognizerSet" 
    recongizers:[ 
        "dialogname.lu"
   ] 
} 

This change is to support the empty recognizer scenario in CrossTrainedRecognizer.

Specific Changes

Testing

Changed projects

• Microsoft.Bot.Builder.Dialogs.Adaptive
• Microsoft.Bot.Builder.Dialogs.Adaptive.Tests