[nodejs parity] Allow LUIS endpoint to be null or empty

[daveta]

Fixes #1387

Enable empty endpoint (just like nodejs)!

[coveralls]

Pull Request Test Coverage Report for Build 53309

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.004%) to 76.126%

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Totals
Change from base Build 53133:	0.004%
Covered Lines:	4362
Relevant Lines:	5730

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💛 - Coveralls

[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
No issues found in Microsoft.Bot.Connector.dll
No issues found in Microsoft.Bot.Schema.dll