Fix map z-level generation starting position (#92836)

## About The Pull Request
The `(x, y, size)` arguments for `CHORDS_TO_1D` were in the wrong
position when a map grid layout is procedurally generated. This results
in z-levels that will likely be clustered together near the edge instead
of spreading out evenly from the center.

## Why It's Good For The Game
Code works as intended now.

## Changelog
🆑
fix: Fix map z-level generation starting position. Z-levels used to
cluster together near the edges and will now instead be spread out
evenly from the center.
/🆑

code/modules/mapping/space_management/space_transition.dm

@@ -161,7 +161,8 @@
 		point.set_neigbours(grid, grid_diameter)
 
 	var/center = round(grid_diameter / 2)
-	point = grid[CHORDS_TO_1D(grid_diameter, center, center)]
+	if(transition_levels.len)
+		point = grid[CHORDS_TO_1D(center, center, grid_diameter)]
 	grid.Cut()
 
 	var/list/transition_pick = transition_levels.Copy()