Add null conditional operators to helmet code (#5660)

# About the pull request

So I am not able to reproduce the runtime spam errors reported in #5656
prompting #5655 but I have added null conditional operators where it is
complaining.

# Explain why it's good for the game

Less runtimes? Fixes #5656

# Testing Photographs and Procedure
Untested other than confirming it still seems fine? Please let me know
if you know how to reproduce it.

# Changelog
:cl: Drathek
fix: Added more null testing to helmet visors.
/:cl:

code/modules/clothing/head/helmet.dm

@@ -677,7 +677,7 @@ GLOBAL_LIST_INIT(allowed_helmet_items, list(
 				if(length(total_visors) > iterator)
 					var/obj/item/device/helmet_visor/next_visor = total_visors[iterator + 1]
 
-					if(!isnull(GLOB.huds[next_visor.hud_type].hudusers[user]))
+					if(!isnull(GLOB.huds[next_visor.hud_type]?.hudusers[user]))
 						iterator++
 						skipped_hud = TRUE
 						continue
@@ -694,7 +694,7 @@ GLOBAL_LIST_INIT(allowed_helmet_items, list(
 
 	for(var/obj/item/device/helmet_visor/new_visor in total_visors)
 
-		if(!isnull(GLOB.huds[new_visor.hud_type].hudusers[user]))
+		if(!isnull(GLOB.huds[new_visor.hud_type]?.hudusers[user]))
 			continue
 
 		active_visor = new_visor