This is really a case of careless driving on your part unfortunately,
there is no real defence for hitting a parked car unless that vehicle is parked illegally or dangerously. Having said that
I suppose it could be worth getting the opinion of your insurance company on the grounds that the othe car was 'trespassing' by being partly on your property without permission, You could argue that you did not bother to look in your wing mirror on that side as there should not be anything there.
Worth a try?