Use a broker who can do this – don’t bother going to insurers. Usual insurance company rules are either 30/60 day period before it’s classed as unoccupied, but as it presumably already is and will be for some time then you need to get advice.
People will try and get around it by having the property “inspected” weekly, but the wording is usually that the property will be used for overnight accommodation by someone. Don’t take the chance – insure it properly.
Bear in mind that cover is usually restricted quite a bit – basically it’s natural perils only (storm/flood/etc) – so it’s often worth making sure there’s literally nothing of value in the property if you’re worried about theft.