Fixes CI runtime/race condition with forensics (tgstation#81610)

## About The Pull Request

Just a quick fix for the following CI runtime, and additionally passes
through the rest of this file's weakref instances for any other
potential bugs.


![image](https://github.com/tgstation/tgstation/assets/13398309/a58be18a-21e4-40b2-92e0-1a961108c939)

## Why It's Good For The Game

Bugfix

## Changelog

Nothing player facing

---------

Co-authored-by: san7890 <[email protected]>

code/modules/forensics/_forensics.dm

@@ -70,6 +70,7 @@
 	var/atom/parent_atom = parent.resolve()
 	if (!isnull(parent_atom))
 		UnregisterSignal(parent_atom, list(COMSIG_COMPONENT_CLEAN_ACT))
+	parent = null
 	return ..()
 
 /// Empties the fingerprints list
@@ -147,7 +148,9 @@
 /// Adds a single fiber
 /datum/forensics/proc/add_fibers(mob/living/carbon/human/suspect)
 	var/fibertext
-	var/atom/actual_parent = parent.resolve()
+	var/atom/actual_parent = parent?.resolve()
+	if(isnull(actual_parent))
+		parent = null
 	var/item_multiplier = isitem(actual_parent) ? ITEM_FIBER_MULTIPLIER : NON_ITEM_FIBER_MULTIPLIER
 	if(suspect.wear_suit)
 		fibertext = "Material from \a [suspect.wear_suit]."
@@ -214,8 +217,11 @@
 		if(last_stamp_pos)
 			LAZYSET(hiddenprints, suspect.key, copytext(hiddenprints[suspect.key], 1, last_stamp_pos))
 		hiddenprints[suspect.key] += "\nLast: \[[current_time]\] \"[suspect.real_name]\"[has_gloves]. Ckey: [suspect.ckey]" //made sure to be existing by if(!LAZYACCESS);else
-	var/atom/parent_atom = parent.resolve()
-	parent_atom.fingerprintslast = suspect.ckey
+	var/atom/parent_atom = parent?.resolve()
+	if(!isnull(parent_atom))
+		parent_atom.fingerprintslast = suspect.ckey
+	else
+		parent = null
 	return TRUE
 
 /// Adds the given list into blood_DNA
@@ -230,7 +236,10 @@
 
 /// Updates the blood displayed on parent
 /datum/forensics/proc/check_blood()
-	var/obj/item/the_thing = parent.resolve()
+	var/obj/item/the_thing = parent?.resolve()
+	if(isnull(the_thing))
+		parent = null
+		return
 	if(!istype(the_thing) || isorgan(the_thing)) // organs don't spawn with blood decals by default
 		return
 	if(!length(blood_DNA))