Remove (extra) args from poll `Checkin` call

Currently the call to checkin within the polling loop uses extra args that get stored within the bulk checkin's `extraT` struct. Having anything in this struct prevents the bulk checkin handler from re-using serializations for different agents; these args should be removed.

https://github.com/elastic/fleet-server/blob/898abd59e2fe81e0ffe5a4dd060bf89396d326a3/internal/pkg/api/handleCheckin.go#L386-L389

We should also test removing all optional args in this invocation as the purpose of the loop `Checkin` is to update the timestamps, not any of the other attributes