TGUI Alert Button List Cleanup (#22207)

* Please describe the intent of your changes in a clear fashion.
Fixes
[SERVER-PROD-4](https://sentry.io/organizations/aurorastation/issues/7404427517/).
The issue was that the `QDEL_NULL` macro was used on a list variable,
causing `qdel` to error when a list was passed directly. To resolve
this, `QDEL_NULL(buttons)` was replaced with `buttons = null` in
`/datum/tgui_alert/Destroy`. This refactors the cleanup of the buttons
list during alert destruction. This fix was generated by Seer in Sentry,
triggered by Werner. 👁️ Run ID: 13170119. Not quite right? [Click here
to continue debugging with
Seer.](https://sentry.io/organizations/aurorastation/issues/7404427517/?seerDrawer=true)
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images of these changes in the PR's description.
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comment with !wip or !review required
* If you include sprites/sounds/... (assets) that you have not created
yourself specify the license and original author below.
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changelog as specified in the contributor guidelines

### Asset Licenses
The following assets that **have not** been created by myself are
included in this PR:

| Path | Original Author | License |
| --- | --- | --- |
| icons/example.dmi | ExamplePerson (Example Station) | CC0 |

---------

Signed-off-by: Arrow768 <1331699+Arrow768@users.noreply.github.com>
Co-authored-by: sentry[bot] <39604003+sentry[bot]@users.noreply.github.com>
Co-authored-by: Arrow768 <1331699+Arrow768@users.noreply.github.com>

code/modules/tgui_input/alert.dm

@@ -72,7 +72,7 @@
 
 /datum/tgui_alert/Destroy(force, ...)
 	SStgui.close_uis(src)
-	QDEL_NULL(buttons)
+	QDEL_LIST(buttons)
 	return ..()
 
 /**