Fix linter errors

lib/util/utils.ts

@@ -23,6 +23,7 @@ export function responseToBody(httpOperationResponseMethod: Function, ...args: a
   const callback = args[args.length - 1];
   if (typeof callback === "function") {
     httpOperationResponseMethod(...args.slice(0, args.length - 1))
+      // tslint:disable-next-line:no-null-keyword
       .then((res: HttpOperationResponse) => callback(null, res.parsedBody, res.request, res))
       .catch((err: any) => callback(err));
   } else {