An unknown aircraft on the wrong flight path and most likely squawking the wrong IFF codes isn't getting into Israeli airspace without the IDF having an opinion about it. Especially if an aircraft has gone missing anywhere in the world.
I'm not saying Israel can't be nuked, but it would be very, very difficult to do it by aircraft. They are on the lookout for such things 24/7.
I didn't suggest an unknown aircraft. A regularly scheduled, passenger filled, aircraft, with a nuke in its cargo hold.
As for their defenses, I hope they do look out for such things and am sure they do. The bad guys have to be right only once.