indicate in 'close' or 'beforeclose' that initial focus element no longer exists

The fix for #8730 introduced focus handling for closing a dialog. However, if the dialog was opened from an option in a menu, then the element that had focus when it was opened no longer exists in the DOM when it is closing. There should be some sort of indicator for devs to pick up in the 'close' or 'beforeclose' event that indicates the focus element no longer exists so that they can manage focus instead of allowing focus to go to the body element.