Fixes ashwalker lung runtimes on runtimestation (#64707)

I've been getting this error every time I launch a local server on runtimestation with LOWMEMORYMODE for months and months now from one of the ashwalker dummies. I'm now finally getting around to fixing it so I don't have to see this same damn error all the time.

code/modules/surgery/organs/lungs.dm

@@ -585,6 +585,10 @@
 	. = ..()
 
 	var/datum/gas_mixture/immutable/planetary/mix = SSair.planetary[LAVALAND_DEFAULT_ATMOS]
+
+	if(!mix?.total_moles()) // this typically means we didn't load lavaland, like if we're using #define LOWMEMORYMODE
+		return
+
 	// Take a "breath" of the air
 	var/datum/gas_mixture/breath = mix.remove(mix.total_moles() * BREATH_PERCENTAGE)