Can I sit in a seat that is reserved, but unoccupied?
Yes. You can sit in a seat that is reserved, but unoccupied. If the seat is still empty a few minutes after departing the station it’s reserved from, go for it! You are allowed to sit in unoccupied reserved seats even if you don’t have any seat reservation. But make sure the seat is in the same class of accommodation for your ticket type.
More details on seat reservations are available here.