Spacer on bottom visible staff of system with hidden staves should act as a system spacer

1. Open score: //musescore.org/sites/musescore.org/files/issues/11bugs.mscz
2. Style->Edit General Style, turn on Hide empty staves
3. Drag a spacer to the first bar of the second system
4. Double click it and extend it downward

Result: no space is created
Expected result: space is created

The problem is, the trumpet staff (which is the only one now visible) is not actually the last staff of the system. So the spacer is presumably creating space *between* the staves of the system, not between the systems. But this is rather counterintuitive. Only workaround is to add the spacer to the bottom staff of the system *before* hiding the empty staves, but that's not a view in which you necessarily know how much space you need. And any spacer added that way is inaccessible when "Hide empty staves" is turned on, although its effects remain.