I was just there in early March and was told by CMs at three different restaurants that they are not charging if your party is short of the number the ADR is for. We had an ADR for 6 at Cape May and only 5 came (MIL wasn't feeling up to going out for dinner) and it was no problem.
Now, if you know the week before your trip that it will be 6 instead of 7, you may want to call the dining line and get them to adjust all your ADRs down to 6, just to be safe. And if you get a CM who says they have to cancel the ADRs instead of just adjusting them, hang up and call back to get another CM.