Fix baseturf helper (#40281)

Would cause issues with turfs where all their baseturfs got removed.

Also switches airless asteroid baseturf to itself.

code/game/turfs/simulated/floor/plating/asteroid.dm

@@ -121,6 +121,7 @@
 
 /turf/open/floor/plating/asteroid/airless
 	initial_gas_mix = "TEMP=2.7"
+	baseturfs = /turf/open/floor/plating/asteroid/airless
 	turf_type = /turf/open/floor/plating/asteroid/airless
 
 

code/modules/mapping/mapping_helpers.dm

@@ -39,12 +39,17 @@
 		for(var/i in baseturf_cache)
 			if(baseturf_to_replace[i])
 				baseturf_cache -= i
+		if(!baseturf_cache.len)
+			thing.assemble_baseturfs(baseturf)
+		else
+			thing.PlaceOnBottom(null, baseturf)
 	else if(baseturf_to_replace[thing.baseturfs])
 		thing.assemble_baseturfs(baseturf)
-		return
-
+	else
 		thing.PlaceOnBottom(null, baseturf)
 
+	
+
 /obj/effect/baseturf_helper/space
 	name = "space baseturf editor"
 	baseturf = /turf/open/space