Fix Detach and on_detach parameter oversights (#5470)

# About the pull request

This PR fixes some confusion with Detatch logic for gun attachments. The
base implementation didn't seem to originally have a user variable but
all other implementations did.

# Explain why it's good for the game

Less pitfalls for coders.

# Testing Photographs and Procedure
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# Changelog
:cl: Drathek
code: Cleaned up some oversights in attachment Detatch logic
/:cl:

code/modules/projectiles/gun_attachables.dm

@@ -168,7 +168,8 @@ Defined in conflicts.dm of the #defines folder.
 /obj/item/attachable/proc/Detach(mob/user, obj/item/weapon/gun/detaching_gub)
 	if(!istype(detaching_gub)) return //Guns only
 
-	detaching_gub.on_detach(user)
+	if(user)
+		detaching_gub.on_detach(user, src)
 
 	if(flags_attach_features & ATTACH_ACTIVATION)
 		activate_attachment(detaching_gub, null, TRUE)

code/modules/projectiles/gun_helpers.dm

@@ -395,7 +395,7 @@ DEFINES in setup.dm, referenced here.
 			playsound(user, 'sound/handling/attachment_add.ogg', 15, 1, 4)
 			return TRUE
 
-/obj/item/weapon/gun/proc/on_detach(obj/item/attachable/attachment)
+/obj/item/weapon/gun/proc/on_detach(mob/user, obj/item/attachable/attachment)
 	return
 
 /obj/item/weapon/gun/proc/update_attachables() //Updates everything. You generally don't need to use this.