fix hyper-noblium formation when bz is null (#93193)

## About The Pull Request
Hyper-Noblium formation is able to occur with or without BZ as a
moderator. The BZ-less formation reaction is commonly used in Ordnance
for high-yield tests. Currently, it throws an error when attempting to
perform the reaction without BZ present because it attempts a list
lookup on the null BZ variable. This results in the TTV failing to
detonate when the valve opens, which is capable of creating impossible
tank states.
<img width="334" height="199" alt="image"
src="https://github.com/user-attachments/assets/74b689c1-9566-47e7-8378-6c3aeeb50be2"
/>

I believe issue #93064 is actually a result of the error this PR fixes.
Comments in that issue also mention a case that I have encountered,
where the TTV was signaled, failed to detonate, and then detonated
instantly in-hand when one of the tanks was removed. Those comments also
mention the Blast Cannon not working with the hyper-noblium reaction,
which is exactly what prompted me to make this fix, and is working with
this fix in place.
## Why It's Good For The Game

Misfired TTVs loaded with maxcaps that detonate unexpectedly is quite
funny, but also very frustrating for the player who properly constructed
the device.
## Changelog
🆑
fix: fixed error during hyper-noblium formation reaction when bz is not
present
/🆑

---------

Co-authored-by: Pickle-Coding <58013024+Pickle-Coding@users.noreply.github.com>

code/modules/atmospherics/gasmixtures/reactions.dm

@@ -815,8 +815,8 @@
 	var/list/tritium = cached_gases[/datum/gas/tritium]
 	/// List of gases we will assert, and possibly garbage collect.
 	var/list/asserted_gases = list(/datum/gas/hypernoblium, /datum/gas/bz)
-	var/list/bz = cached_gases[/datum/gas/bz]
 	air.assert_gases(arglist(asserted_gases))
+	var/list/bz = cached_gases[/datum/gas/bz]
 	var/reduction_factor = clamp(tritium[MOLES] / (tritium[MOLES] + bz[MOLES]), 0.001 , 1) //reduces trit consumption in presence of bz upward to 0.1% reduction
 	var/nob_formed = min((nitrogen[MOLES] + tritium[MOLES]) * 0.01, tritium[MOLES] * INVERSE(5 * reduction_factor), nitrogen[MOLES] * INVERSE(10))