[Regression] Spacers start adding space between systems too soon

This is an issue in 3.1 RC:

1) default empty score
2) add spacer down to measure 2
3) extend downward towards the next system

Result: it starts adding space as soon as you pass the minimum system distance (8.5sp default), rather than waiting until you reach the top staff of the next system.

This is due to the change I made for #285467: System distance unequal when system heights differ. I understand what is going on, and I believe I have a fix.