Fixes minor runtime with null skills pushups (#4680)

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Doing push up with null skills runtimes at every iteration since pushups
don't account for it

I wouldn't normally bother but I came across an unreadable log spammed
by zombie pushups, because our zombie players LOVE doing pushups

No functional changes, no changelog, etc

code/modules/mob/living/carbon/human/exercise.dm

@@ -102,7 +102,7 @@ Verbs related to getting fucking jacked, bro
 	//humans have 100 stamina
 	//default loss per pushup = 5 stamina
 	var/stamina_loss = 5
-	if(issynth(src))
+	if(!skills || issynth(src))
 		return 0
 	switch(skills.get_skill_level(SKILL_ENDURANCE))
 		if(SKILL_ENDURANCE_NONE)