Harden Prompts

[mdrichardson]

Fixes microsoft/BotBuilder-Samples#1728
Fixes: #2271

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

if (!utterance) {
  return result;
}

to:

1. ChoicePrompt
2. DatetimePrompt
3. NumberPrompt

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

Testing

Tested with CoreBot and it fixed original issue.

Note

I'm currently checking to see if the other SDK languages need something like this.

[coveralls]

Pull Request Test Coverage Report for Build 151204

• 6 of 7 (85.71%) changed or added relevant lines in 4 files are covered.
• 9 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.07%) to 81.95%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
libraries/botbuilder-dialogs/src/prompts/numberPrompt.ts	1	2	50.0%

Files with Coverage Reduction	New Missed Lines	%
libraries/botframework-streaming/src/webSocket/webSocketTransport.ts	2	80.65%
libraries/botframework-streaming/src/webSocket/webSocketServer.ts	7	66.67%

Totals
Change from base Build 151130:	-0.07%
Covered Lines:	14557
Relevant Lines:	16888

---

💛 - Coveralls

[mdrichardson]

@GeoffCoxMSFT Good suggestions. Made the changes and am requesting re-review.