[MIRROR] Nullchecks Client on tgui_alert() and siblings [MDB IGNORE] (#24703)

* Nullchecks Client on `tgui_alert()` and siblings (#79322)

## About The Pull Request

Fixes #79321

```dm
/datum/tgui/proc/open()
	if(!user.client)
		return FALSE
```
The TGUI window won't even `open()` and do any work if we don't have a
client, so let's just explicitly get the hell out of dodge if we don't
have a client associated with a mob in these procs. Adding a `?` to
handle the runtime in the linked issue only obfuscates the deeper issue
because of the aforementioned code snippet.

## Why It's Good For The Game

Clientless monkeys will still somehow be able to interact with stuff
through their random behavior, and this is still plausible enough to
show up on live servers every so often, so let's just patch it out
early. These alerts are meant for player user input, so if we don't have
a player, let's bounce.
## Changelog
A player would never ever notice this.

* Nullchecks Client on `tgui_alert()` and siblings

---------

Co-authored-by: san7890 <the@san7890.com>

code/modules/tgui_input/alert.dm

@@ -18,7 +18,11 @@
 			var/client/client = user
 			user = client.mob
 		else
-			return
+			return null
+
+	if(isnull(user.client))
+		return null
+
 	// A gentle nudge - you should not be using TGUI alert for anything other than a simple message.
 	if(length(buttons) > 3)
 		log_tgui(user, "Error: TGUI Alert initiated with too many buttons. Use a list.", "TguiAlert")

code/modules/tgui_input/checkboxes.dm

@@ -14,13 +14,17 @@
 	if (!user)
 		user = usr
 	if(!length(items))
-		return
+		return null
 	if (!istype(user))
 		if (istype(user, /client))
 			var/client/client = user
 			user = client.mob
 		else
-			return
+			return null
+
+	if(isnull(user.client))
+		return null
+
 	if(!user.client.prefs.read_preference(/datum/preference/toggle/tgui_input))
 		return input(user, message, title) as null|anything in items
 	var/datum/tgui_checkbox_input/input = new(user, message, title, items, min_checked, max_checked, timeout, ui_state)

code/modules/tgui_input/list.dm

@@ -14,13 +14,17 @@
 	if (!user)
 		user = usr
 	if(!length(items))
-		return
+		return null
 	if (!istype(user))
 		if (istype(user, /client))
 			var/client/client = user
 			user = client.mob
 		else
-			return
+			return null
+
+	if(isnull(user.client))
+		return null
+
 	/// Client does NOT have tgui_input on: Returns regular input
 	if(!user.client.prefs.read_preference(/datum/preference/toggle/tgui_input))
 		return input(user, message, title, default) as null|anything in items

code/modules/tgui_input/number.dm

@@ -23,7 +23,11 @@
 			var/client/client = user
 			user = client.mob
 		else
-			return
+			return null
+
+	if (isnull(user.client))
+		return null
+
 	// Client does NOT have tgui_input on: Returns regular input
 	if(!user.client.prefs.read_preference(/datum/preference/toggle/tgui_input))
 		var/input_number = input(user, message, title, default) as null|num

code/modules/tgui_input/text.dm

@@ -23,7 +23,11 @@
 			var/client/client = user
 			user = client.mob
 		else
-			return
+			return null
+
+	if(isnull(user.client))
+		return null
+
 	// Client does NOT have tgui_input on: Returns regular input
 	if(!user.client.prefs.read_preference(/datum/preference/toggle/tgui_input))
 		if(encode)