Prevent from server down when request body has nothing
Prevent from server down when request body has nothing

How to reproduce
- Build a bot with 'botbuilder' and 'express'
- Send a request without body
- Server down

Sample request:
% curl -X POST http://localhost:3978/api/messages

Stack trace
SyntaxError: Unexpected end of JSON input
    at Object.parse (native)
    at IncomingMessage.<anonymous> (botbuilder/lib/bots/ChatConnector.js:43:37)
    at emitNone (events.js:86:13)
    at IncomingMessage.emit (events.js:185:7)
    at endReadableNT (_stream_readable.js:975:12)
    at _combinedTickCallback (internal/process/next_tick.js:74:11)
    at process._tickCallback (internal/process/next_tick.js:98:9)

Reference:
https://nodejs.org/en/docs/guides/anatomy-of-an-http-transaction/#header-request-body

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