Fixes persisting brimbeams (#89318)

## About The Pull Request

If a brimdemon gets destroyed instantly, their beams would persist
because of a runtime here as owner in actions can be null.

Fixes https://github.com/tgstation/tgstation/issues/81994

## Changelog
🆑
fix: Fixed an issue with persisting brimbeams
/🆑

code/modules/mob/living/basic/lavaland/brimdemon/brimbeam.dm

@@ -86,7 +86,8 @@
 /datum/action/cooldown/mob_cooldown/brimbeam/proc/extinguish_laser()
 	if(!length(beam_parts))
 		return FALSE
-	owner.move_resist = initial(owner.move_resist)
+	if (owner)
+		owner.move_resist = initial(owner.move_resist)
 	for(var/obj/effect/brimbeam/beam in beam_parts)
 		beam.disperse()
 	beam_parts = list()