Fixes a few runtimes from HUD cycling

[SabreML]

About the pull request

Fixes a few runtimes from the 'Cycle Helmet HUD' keybind.
Specifically:

---

runtime error: undefined proc or verb /obj/item/clothing/head/beret/cm/cycle huds().
 - 
 - proc name: down (/datum/keybinding/human/cycle_helmet_hud/down)
 -   source file: human.dm,125
 -   usr: Sean Agg (/mob/living/carbon/human)
 -   src: cycle_helmet_hud (/datum/keybinding/human/cycle_helmet_hud)
 -   usr.loc: the floor (48,79,2) (/turf/open/floor/almayer)
 -   call stack:
 - cycle_helmet_hud (/datum/keybinding/human/cycle_helmet_hud): down(SabreML (/client))
 - SabreML (/client): keyDown("J")

Caused if the player was wearing a hat which wasn't a /obj/item/clothing/head/helmet/marine subtype.
(In this case a beret)

---

runtime error: Cannot read null.vars
 - proc name: down (/datum/keybinding/human/cycle_helmet_hud/down)
 -   source file: human.dm,127
 -   usr: Sean Agg (/mob/living/carbon/human)
 -   src: cycle_helmet_hud (/datum/keybinding/human/cycle_helmet_hud)
 -   usr.loc: the floor (48,78,2) (/turf/open/floor/almayer)
 -   call stack:
 - cycle_helmet_hud (/datum/keybinding/human/cycle_helmet_hud): down(SabreML (/client))
 - SabreML (/client): keyDown("J")

Caused if the player wasn't wearing a hat at all.

---

runtime error: Cannot execute null.set action overlay().
 - proc name: down (/datum/keybinding/human/cycle_helmet_hud/down)
 -   source file: human.dm,128
 -   usr: Sean Agg (/mob/living/carbon/human)
 -   src: cycle_helmet_hud (/datum/keybinding/human/cycle_helmet_hud)
 -   usr.loc: the floor (49,77,2) (/turf/open/floor/almayer)
 -   call stack:
 - cycle_helmet_hud (/datum/keybinding/human/cycle_helmet_hud): down(SabreML (/client))
 - SabreML (/client): keyDown("J")

Caused if the player was wearing a /obj/item/clothing/head/helmet/marine subtype, but it didn't have a HUD action.
(In this case the reporter helmet)

---

I should note that while this is mostly adding extra checks, I did remove the ? from human_user?.head check. As far as I know it isn't possible for a client not to have an attached mob, and /datum/keybinding/human/can_use() covers that either way.

Explain why it's good for the game

Three bugfixes for the price of one!

Testing Photographs and Procedure

Screenshots & Videos

Put screenshots and videos here with an empty line between the screenshots and the <details> tags.

Changelog

🆑
fix: Fixed a few runtime errors caused by the 'Cycle Helmet HUD' keybind.
/:cl: