Fixes damp rag runtime (#93192)

## About The Pull Request

<img width="564" height="75" alt="image"
src="https://github.com/user-attachments/assets/539de6b9-4c4e-40cd-9ebf-d891db25ede2"
/>

Seems to be caused by add_blood_dna() being passed an empty list (which
happens by default with rags).

<details><summary> see here </summary>

<img width="591" height="518" alt="image"
src="https://github.com/user-attachments/assets/9c1e6863-8707-4768-87ac-a2e0874fd34d"
/>

`all_blood_dna` is not null but an empty list to start, so we need our
proc to be able to handle such cases

</details>

So anytime you clean something that doesn't have blood dna you will get
this runtime.

## Why It's Good For The Game

If someone is cleaning these really can build up. Less runtime log
pollution so we notice important issues.

## Changelog

N/A

code/modules/reagents/reagent_containers/rag.dm

@@ -145,7 +145,8 @@
 			fire_act(500, 100)
 
 	// FINALLY add the dna to us
-	add_blood_DNA(all_blood_dna)
+	if(length(all_blood_dna))
+		add_blood_DNA(all_blood_dna)
 	update_appearance()
 	if(blood_level >= 10)
 		to_chat(cleaner, span_warning("[src] is too dirty to clean anything else! Wash it first!"))