Add null checks to strip_appearance_underlays and copy_appearance_filter_overlays (#95474)

## About The Pull Request

This adds null checks to `strip_appearance_underlays` and
`copy_appearance_filter_overlays`, because I have seen many runtimes on
live from null overlays/underlays existing - and the runtimes mean that
the procs will return null, potentially resulting in a strange blank
icon for whatever's using it.

Also added return type hints to those two procs, 'cuz why not.

## Why It's Good For The Game

I'd rather have these procs consistently work.

## Changelog

No user-facing changes, prolly. Might fix bugs, idk which in specific.

code/__HELPERS/icons.dm

@@ -1335,9 +1335,11 @@ GLOBAL_LIST_EMPTY(transformation_animation_objects)
 
 /// Strips all underlays on a different plane from an appearance.
 /// Returns the stripped appearance.
-/proc/strip_appearance_underlays(mutable_appearance/appearance)
+/proc/strip_appearance_underlays(mutable_appearance/appearance) as /mutable_appearance
 	var/base_plane = PLANE_TO_TRUE(appearance.plane)
 	for(var/mutable_appearance/underlay as anything in appearance.underlays)
+		if(isnull(underlay))
+			continue
 		if(PLANE_TO_TRUE(underlay.plane) != base_plane)
 			appearance.underlays -= underlay
 	return appearance
@@ -1348,13 +1350,15 @@ GLOBAL_LIST_EMPTY(transformation_animation_objects)
  * Filters out certain overlays from the copy, depending on their planes
  * Prevents stuff like lighting from being copied to the new appearance
  */
-/proc/copy_appearance_filter_overlays(appearance_to_copy)
+/proc/copy_appearance_filter_overlays(appearance_to_copy) as /mutable_appearance
 	var/mutable_appearance/copy = new(appearance_to_copy)
 	var/static/list/plane_whitelist = list(FLOAT_PLANE, GAME_PLANE, FLOOR_PLANE)
 
 	/// Ideally we'd have knowledge what we're removing but i'd have to be done on target appearance retrieval
 	var/list/overlays_to_keep = list()
 	for(var/mutable_appearance/special_overlay as anything in copy.overlays)
+		if(isnull(special_overlay))
+			continue
 		var/mutable_appearance/real = new()
 		real.appearance = special_overlay
 		if(PLANE_TO_TRUE(real.plane) in plane_whitelist)
@@ -1363,6 +1367,8 @@ GLOBAL_LIST_EMPTY(transformation_animation_objects)
 
 	var/list/underlays_to_keep = list()
 	for(var/mutable_appearance/special_underlay as anything in copy.underlays)
+		if(isnull(special_underlay))
+			continue
 		var/mutable_appearance/real = new()
 		real.appearance = special_underlay
 		if(PLANE_TO_TRUE(real.plane) in plane_whitelist)