Fix min_by? in IOCP event loop #run_once

[straight-shoota]

min_by raises if the queue is empty but we want to handle this case explicitly in the following condition.

This is practically not very relevant as the queue should never be empty in normal operation. But it might be useful when working on the event loop itself.

[ysbaddaden]

The current Crystal::Scheduler happen to always have an event in the event loop, but that may not be true in the future anymore (e.g. RFC #2).