Handle null message.Text in DateTimePrompt

[xieofxie]

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Is your feature request related to a problem? Please describe.
DateTimeRecognizer.RecognizeDateTime itself doesn't handle null query:

botbuilder-dotnet/libraries/Microsoft.Bot.Builder.Dialogs/Prompts/DatetimePrompt.cs

Line 104 in b08e2fb

var results = DateTimeRecognizer.RecognizeDateTime(message.Text, culture, refTime: refTime);

It will throw

System.NullReferenceException: Object reference not set to an instance of an object.
  Stack Trace: 
    QueryProcessor.Preprocess(String query, Boolean caseSensitive, Boolean recode)
    DateTimeModel.Parse(String query, DateTime refTime)
    DateTimeRecognizer.RecognizeDateTime(String query, String culture, DateTimeOptions options, Nullable`1 refTime, Boolean fallbackToDefaultCulture)
    DateTimePrompt.OnRecognizeAsync(ITurnContext turnContext, IDictionary`2 state, PromptOptions options, CancellationToken cancellationToken) line 104

Describe the solution you'd like
Handle null in DateTimePrompt

Describe alternatives you've considered
Handle in DateTimeRecognizer.RecognizeDateTime

Additional context
Easily happen in DLS

[enhancement]

[EricDahlvang]

Looks like @mdrichardson has already completed this work: #3949

Thanks @mdrichardson !