callOnEachEvent throws if eventTypes is null

I read the code here:

https://github.com/zulip/zulip-js/blob/cee33876ce4965ff1f17491b79527ff39ce4d8a0/src/events_wrapper.js#L42

I assume that the second parameter was optional, but omitting it produced a firehose of errors. Adding it back as an empty array stopped the errors. I'd be happy to submit a PR switching `null` to `[]` if that's helpful. Seems like `null` is not a valid value.