Fix two gas assertion runtimes (tgstation#53718)

/datum/gas_mixture/proc/remove(amount) has a code path that can return null.

All the code following the runtimes assume some bit of gas_mixture was actually removed at all. No need to trigger this code if there's no gas_mixture removed, can early return instead.

code/game/machinery/electrolyzer.dm

@@ -92,6 +92,10 @@
 
 	var/datum/gas_mixture/env = L.return_air() //get air from the turf
 	var/datum/gas_mixture/removed = env.remove(0.1 * env.total_moles())
+
+	if(!removed)
+		return
+
 	removed.assert_gases(/datum/gas/water_vapor, /datum/gas/oxygen, /datum/gas/hydrogen)
 	var/proportion = min(removed.gases[/datum/gas/water_vapor][MOLES], (1.5 * delta_time * workingPower))//Works to max 12 moles at a time.
 	removed.gases[/datum/gas/water_vapor][MOLES] -= proportion * 2 * workingPower

code/modules/events/crystal_event.dm

@@ -363,6 +363,10 @@ This section is for the destabilized SM
 	var/datum/gas_mixture/removed
 	var/gasefficency = 0.5
 	removed = env.remove(gasefficency * env.total_moles())
+
+	if(!removed)
+		return
+
 	removed.assert_gases(/datum/gas/bz, /datum/gas/miasma)
 	if(!removed || !removed.total_moles() || isspaceturf(loc_turf))
 		removed.gases[/datum/gas/bz][MOLES] += 0.5