I think that is acceptable, i.e. `std` does the same, and we would add the warning as well.

To be honest, I would guess that the empty special case was just part of many such added when the general support for this was much worse, and the test should just be changed. As far as I can see, the current result is almost always incorrect. Since the old result is plain wrong, I would consider it a bug fix and probably no big warnings are needed. But we could of course add a FutureWarning for now...