Add guard to ensure that body is non null

TypeScript was complianing a little later on when we tried to
Buffer.from `event.body` since it may be null.

aws-ts-slackbot/index.ts

@@ -76,7 +76,7 @@ const endpoint = new awsx.apigateway.API("mentionbot", {
  throw new Error("mentionbot:verificationToken was not provided")
  }
 
- if (!event.isBase64Encoded) {
+ if (!event.isBase64Encoded || event.body == null) {
  console.log("Unexpected content received");
  console.log(JSON.stringify(event));
  }