Restore .offset disconnected/invalid behavior

As of jQuery 3.0, disconnected or non-element input to `.offset` will no longer return `{ top: 0, left: 0 }`, and may throw. Migrate should restore functionality changed by https://github.com/jquery/jquery/commit/1617479fcf7cbdaf33dc9334ed10a0f30bf14687 / https://github.com/jquery/jquery/commit/d4dd548acaf049d8e4dca9e4b879a51f26bb3d27, and warn when there is no layout box.
