[PORT] Hardened prompts

[tomlm]

Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#3949

See parent PR: #2246

Description

Some of the prompts break if user uploads an attachment instead of responding with text because of some of the string methods that get called on activity.text (which doesn't exist if an attachment is sent).

Specific Changes

Add some variation of

if (string.isNullOrEmpty(turnContext.Activity.Text)) {
  return Task.FromResult(result);
}

to:

1. ChoicePrompt
2. DatetimePrompt
3. NumberPrompt
4. ConfirmPrompt

I also renamed message to utterance to be more in line with other prompts.

The original issue was only for DatetimePrompt, but I added the other two. Other prompts didn't seem relevant for this.

Changed projects

• Microsoft.Bot.Builder.Dialogs

[mdrichardson]

Already covered in original PR: #2246