allFlagsState hangs when passing callback but no options

Even though the `options` parameter for `allFlagsState` is supposed to be optional [1], it's actually required when passing a callback:

```js
ldClient.allFlagsState(user, callback); // this hangs
ldClient.allFlagsState(user, {}, callback); // this works
```

[1] https://github.com/launchdarkly/node-client/blob/62903ce09aa3311cecd126baff327958fc075eed/index.d.ts#L550-L569