Mob trophies will no longer attempt to drop if there aren't any (#3116)

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## About The Pull Request

killing 200000 goliaths so I can harvest nothing from them and upgrade
my crusher with no trophies

## Why It's Good For The Game

runtime I think

## Changelog

:cl:
fix: mobs will no longer stick around after being butchered sometimes
/:cl:

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Signed-off-by: Theos <[email protected]>

code/modules/mob/living/simple_animal/hostile/mining_mobs/mining_mobs.dm

@@ -75,7 +75,8 @@
 	..(gibbed)
 
 /mob/living/simple_animal/hostile/asteroid/proc/spawn_mob_trophy()
-	butcher_results[mob_trophy] = 1
+	if(mob_trophy)
+		butcher_results[mob_trophy] = 1
 
 /mob/living/simple_animal/hostile/asteroid/handle_temperature_damage()
 	if(bodytemperature < minbodytemp)