As the owner of the property,liability will rest with you if someone is injured in your property (even if they're trespassing). There are a number of companies on the Market that offer unoccupied property cover,cover is usually restricted to liability & fire cover.
It is not strictly the case that the owner of a property will always be liable if someone is injured on their property, there must always be an element of fault / negligence.
This is particularly the case when the injured person is a trespasser who are owed a lesser duty of care ie...not to injure them intentionally or act with a reckless disregard for them or their property.
That said every case is different and the "reckless disregard" element leaves it open to a Court to find in favour of the trespasser so it would be prudent to insure against this risk.