Typecheck all return_air calls (#15363)

code/game/objects/structures/pit.dm

@@ -62,8 +62,6 @@
 /obj/structure/pit/return_air()
 	if(open && loc)
 		return loc.return_air()
-	else
-		return null
 
 /obj/structure/pit/proc/digout(mob/escapee)
 	var/breakout_time = 1 //2 minutes by default

code/game/objects/structures/transit_tubes.dm

@@ -366,6 +366,7 @@ obj/structure/ex_act(severity)
 //  giving it a chance to mix its internal air supply with the turf it is
 //  currently on.
 /obj/structure/transit_tube_pod/proc/mix_air()
+	if(!loc) return
 	var/datum/gas_mixture/environment = loc.return_air()
 
 	//note that share_ratio assumes both gas mixes have the same volume,