[Swighey]

Actually having thought about this again (and there might still be holes in my thinking, so please challenge it if you see fit):

The rules state that the groups are only chosen after a legal called shot is made. Also, “The eight ball may be called only after the shot on which the shooter’s group has been cleared”

So the order is:
1, a legal shot must be made
2, groups are decided
3, the eight can be made

So, in the unusual and unlikely situation where the table is still open with only one group on the table:

The incoming player can play safe or call a stripe. Essentially, you have a choice. Either try to run the table or play a weak safety that forces your opponent to consider doing the same.

According to the letter of the rules I think that’s correct.