fix: Make getTopScoringIntent return '' instead of undefined

[taicchoumsft]

Fixes Composer#8368. fixes #minor.

Description

There is a minor drift between the dotnet and nodeJs implementation of getTopScoringIntent.

1. In the NodeJS implementation, topIntent is declared but not initialized. In the case of empty intents bag, can return undefined.

   NodeJS Implementation

        let topIntent: string;

   In the dotnet implementation, the topIntent is initialized.
   Dotnet Implementation

         var topIntent = (string.Empty, 0.0d);

2. This later causes an issue in crossTrainedRecognizerSet line 110, where an undefined intent can be set.
   The isRedirect function will eventually be called on the undefined intent and cause the 500 error in the bot.

Note: this entire chain of events is started at the end of a text input, when the recognizers are fired with an empty utterance (not sure why). Orchestrator recognizer will return a RecognizerResult with this shape in the case of empty utterance:

        let recognizerResult: RecognizerResult = {
            text,
            intents: {},
            entities: {},
        };

Both LUIS and QnA fill the intents bag.

The dotnet Orchestrator implementation also returns an empty intents dictionary in the case of an empty utterance so we follow that behavior.

Testing

UT added, manual testing to test for correct behavior.

[coveralls]

Pull Request Test Coverage Report for Build 1035028336

• 2 of 2 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 84.342%

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Totals
Change from base Build 1027112788:	0.2%
Covered Lines:	19685
Relevant Lines:	22143

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💛 - Coveralls