Handle an empty volta body more robustly
N.B. This MR incorporates the Repeat_acknowledge_engraver fix under review in !349 (merged).
Sequential_iterator no longer pre-skips elements that are not OK. This maintains the position of the elements, which is important for the derived class Volta_repeat_iterator.
Volta_repeat_iterator issues commands to create repeat bars if and only if the body music has some duration, including grace duration.
It wouldn't be surprising if empty alternatives were not handled very well. I might investigate that, and it might lead to more changes in the same area, but I think the improvements in this MR can stand on their own.
Closes #6025 (closed).