Empty lyrics shouldn't take up vertical space?

[davidweichiang]

It was pointed out in #1658 that in print books, when a line has no text (lyrics), the space to the next line is decreased. But currently, in such cases the space spacelinestext is still allocated for it. Whatever emerges from #1658, I think it makes sense to remove spacelinestext when there is no text. This would affect existing scores, because the baseline (as currently defined) would move.

[rpspringuel]

This makes sense to me and would make spacelinestext more like translationheight and abovelinestextheight (both of which only play a role when the thing the refer to is present).