Materials And Stack Harddel (#22313)

To clarify I have recently cancelled my subscription to google's coding
agents, so this is my first hard del PR in several weeks that was done
completely by hand. This PR fixes a hard del on Materials (and their
parent Stacks), which was being caused by up to 4 different vars
potentially containing references that were not being cleared during
their Destroy() procs.

I have also tested this by hand, by spawning and deleting stacks, then
waiting to see if the hard del triggers or not.

---------

Signed-off-by: VMSolidus <evilexecutive@gmail.com>

code/modules/materials/material_sheets.dm

@@ -50,6 +50,10 @@
 
 	matter = material.get_matter()
 
+/obj/item/stack/material/Destroy()
+	material = null
+	return ..()
+
 /obj/item/stack/material/get_material()
 	return material