Fixes harddel in particle holders (#77627)

## About The Pull Request

The harddel hunt continues...


![image](https://github.com/tgstation/tgstation/assets/13398309/19fb7446-e667-4e4f-9ab5-b80e02541592)

This stupid mythril coin has come up a bunch of times in our CI recently
and it is starting to annoy me. This should fix it hopefully.

Attempts to get rid of a potential source of harddels in
particle_holder.dm

## Why It's Good For The Game


## Changelog

🆑
fix: fixes a harddel in particle holders
/🆑

code/game/objects/effects/particle_holder.dm

@@ -38,6 +38,7 @@
 
 /obj/effect/abstract/particle_holder/Destroy(force)
 	QDEL_NULL(particles)
+	parent = null
 	return ..()
 
 /// Non movables don't delete contents on destroy, so we gotta do this