Fix sector initialization stack overflow crash.

- Replace changes from 8c7371c5d4
    - Removed debugging prints in skybox generation
    - Reverted change to GetConnnectedZLevels to avoid the chicken-or-egg initialization issue of find_z_levels() requiring knowledge of a sector's z-levels during the attempt to determine a sector's z-levels.
    - Make find_z_levels() always called again so that lazy open space initialization works on landable ships again.
- Instead, allow static configuration of sector map_z to override the default behavior of calling GetConnectedZLevels()

code/modules/overmap/sectors.dm

@@ -5,7 +5,7 @@
 	name = "map object"
 	scannable = TRUE
 
-	var/list/map_z = null
+	var/list/map_z = list()
 	var/list/extra_z_levels //if you need to manually insist that these z-levels are part of this sector, for things like edge-of-map step trigger transitions rather than multi-z complexes
 
 	var/list/initial_generic_waypoints //store landmark_tag of landmarks that should be added to the actual lists below on init.
@@ -30,8 +30,7 @@
 	if(. == INITIALIZE_HINT_QDEL)
 		return
 
-	if(!map_z)			// If map_z is already defined, we don't need to find where we are
-		find_z_levels() // This populates map_z and assigns z levels to the ship.
+	find_z_levels() // This populates map_z and assigns z levels to the ship.
 	register_z_levels() // This makes external calls to update global z level information.
 
 	if(!global.using_map.overmap_z)
@@ -59,7 +58,8 @@
 			. += A
 
 /obj/effect/overmap/visitable/proc/find_z_levels()
-	map_z = GetConnectedZlevels(z)
+	if(!LAZYLEN(map_z)) // If map_z is already populated use it as-is, otherwise start with connected z-levels.
+		map_z = GetConnectedZlevels(z)
 	if(LAZYLEN(extra_z_levels))
 		map_z |= extra_z_levels