Fixes a runtime caused by stacks accidentally merging twice (#26795)

* Fixes empty stacks trying to merge together

* more accurate comment

code/game/objects/items/stacks/stack.dm

@@ -127,6 +127,8 @@
 	// We don't only use istype here, since that will match subtypes, and stack things that shouldn't stack
 	if(!istype(check, merge_type) || check.merge_type != merge_type)
 		return FALSE
+	if(amount <= 0 || check.amount <= 0) // no merging empty stacks that are in the process of being qdel'd
+		return FALSE
 	if(is_cyborg) // No merging cyborg stacks into other stacks
 		return FALSE
 	if(ismob(loc) && !inhand) // no merging with items that are on the mob