Get a cycler's open service event, if any

Return the cycler's current service, or null when it is in service.

A dedicated read rather than making the caller scan the history: that list is ordered newest-created first and paginated, so an event recorded after the fact sorts ahead of an older open one and would eventually hide it — the cycler would read as in service while it is out. At most one open event can exist, so this is exact.