sendToTopic() error when data in undefined

if data is `undefined` or `null` in the Message payload  in this file `node_modules/firebase_admin/lib/messaging/messaging.js` line: 670  will return the error in `forEach`

```
if ('data' in payloadCopy) {
            messaging_internal_1.BLACKLISTED_DATA_PAYLOAD_KEYS.forEach((blacklistedKey) => {
.
.
.
 }

```

must add null check;