Allow LuisRecognizer to handle textless messages

[v-kydela]

Fixes #1349

With this change, LuisRecognizer.RecognizeAsync will no longer throw an exception when the turn context's activity has no text. Instead it will bypass the LUIS endpoint (saving the expense of an API call) and create a fake RecognizerResult with a string.Empty intent. This will allow bots to handle textless messages gracefully when they're not designed to explicitly check if the text is null or empty before calling RecognizeAsync. If the bot has a switch statement with a default clause then that will be triggered, probably responding the same as a None intent. Otherwise, nothing will happen.

[coveralls]

Pull Request Test Coverage Report for Build 52947

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 9 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.09%) to 76.076%

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Files with Coverage Reduction	New Missed Lines	%
/libraries/Microsoft.Bot.Builder.AI.LUIS/LuisRecognizer.cs	9	79.49%

Totals
Change from base Build 52422:	0.09%
Covered Lines:	4350
Relevant Lines:	5718

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💛 - Coveralls

[cleemullins]

Please include a note regarding the JS behavior. We need to keep the two aligned.

[v-kydela]

While the Node SDK does not have this bug, it does behave slightly differently compared to this PR. In Node, the empty utterance is sent to the LUIS endpoint, resulting in a needless endpoint call. Then the returned value is different in that there are no intents in Node, whereas this PR creates a fake intent like V3 did. It's uncertain whether these differences warrant changing the behavior of the Node SDK.

In luisRecognizer.test.js there was already a test that accounts for empty utterances, which confirms the need for this .NET PR since in Node an empty utterance produced no error.

I will amend this PR with your requested change ASAP.

[v-kydela]

Curiously, the test I referred to in luisRecognizer.test.js reflects inaccurate behavior. The LUIS recognizer tests in both SDK's work using a mock HTTP client and so they're not actually calling any real LUIS endpoint. The tests just load files with fake LUIS results that are already built into the projects and then test to see if those fake results are what they've already been made to be. So even though the test says an empty utterance should return a None intent, that's not what really happens when an actual endpoint is used.

[cleemullins]

No issues found in Microsoft.Bot.Builder.dll
No issues found in Microsoft.Bot.Builder.AI.Luis.dll
No issues found in Microsoft.Bot.Builder.AI.QnA.dll
No issues found in Microsoft.Bot.Builder.ApplicationInsights.dll
No issues found in Microsoft.Bot.Builder.Azure.dll
No issues found in Microsoft.Bot.Builder.Dialogs.dll
No issues found in Microsoft.Bot.Builder.TemplateManager.dll
No issues found in Microsoft.Bot.Configuration.dll
Compat issues with assembly Microsoft.Bot.Connector:
MembersMustExist : Member 'System.String Microsoft.Bot.Connector.Authentication.AuthenticationConstants.ToChannelFromBotLoginUrl' does not exist in the implementation but it does exist in the contract.

No issues found in Microsoft.Bot.Schema.dll

[v-kydela]

@cleemullins - You can see in my commits that I've made no changes to the Microsoft.Bot.Connector project. I just had a look in the master branch and it looks like ToChannelFromBotLoginUrl was changed to ToChannelFromBotLoginUrlTemplate in the latest commit. I presume the discrepancy you're seeing would be fixed upon merge.

[chrimc62]

Looks like a good solution to this issue.