fix #14353: lyrics and staff spacers don't work with hide empty staves

Fixes calculation of system height when there are spacers down or lyrics adding to distanceDown for a staff and the staves below are all hidden/empty.  I thought maybe spacers up and/or fret diagrams would have a similar issue, but they don't - the bbox() calculation already accounts for those.

I will say I don't totally understand all the interactions between the calculations involved, and I wouldn't mind if someone reviewed this a bit to be sure I am not doing something stupid.  But it does fix the problem cases, and should not change behavior if the bottom staff is not hidden.