message: 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.
diff: @@ -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));
  }
  