Mice no longer spawn on grilles (#21603)

* Mice no longer spawn on grilles

Fixes #21574. Mice no longer spawn on cables that have dense objects
also on the tile.

Also added the Mice Migration event, used for debug purposes, currently
disabled.

* Uses proc that exists to check for density

code/controllers/subsystem/squeak.dm

@@ -15,8 +15,10 @@ var/datum/subsystem/squeak/SSsqueak
 	NEW_SS_GLOBAL(SSsqueak)
 
 /datum/subsystem/squeak/Initialize(timeofday)
+	trigger_migration()
+
+/datum/subsystem/squeak/proc/trigger_migration(num_mice=10)
 	find_exposed_wires()
-	var/num_mice = 10
 
 	var/mob/living/simple_animal/mouse/M
 	var/turf/proposed_turf
@@ -36,5 +38,7 @@ var/datum/subsystem/squeak/SSsqueak
 
 	var/list/all_turfs = block(locate(1,1,1), locate(world.maxx,world.maxy,1))
 	for(var/turf/open/floor/plating/T in all_turfs)
+		if(is_blocked_turf(T))
+			continue
 		if(locate(/obj/structure/cable) in T)
 			exposed_wires += T