Bandaid the null clients in GLOB.clients (#4700)

# About the pull request

Apparently we are somehow getting nulls into GLOB.clients now, and I
don't know why. Stat panel is the most egregious spammer about this
fact, but a rather slow subsystem will also periodically check too.

# Explain why it's good for the game

We don't need tens of thousands of runtimes.

# Testing Photographs and Procedure
<details>
<summary>Screenshots & Videos</summary>

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

</details>


# Changelog
:cl: Drathek
fix: Bandaided a problem with nulls in GLOB.clients
/:cl:

code/controllers/subsystem/inactivity.dm

@@ -1,20 +1,26 @@
-#define INACTIVITY_KICK 6000 //10 minutes in ticks (approx.)
+#define INACTIVITY_KICK 10 MINUTES
 
 SUBSYSTEM_DEF(inactivity)
 	name = "Inactivity"
-	wait = INACTIVITY_KICK
+	wait = 1 MINUTES
 	flags = SS_NO_INIT | SS_BACKGROUND
 	priority = SS_PRIORITY_INACTIVITY
 	runlevels = RUNLEVELS_DEFAULT|RUNLEVEL_LOBBY
 
 /datum/controller/subsystem/inactivity/fire(resumed = FALSE)
-	if (CONFIG_GET(flag/kick_inactive))
-		for(var/i in GLOB.clients)
-			var/client/C = i
-			if(C.admin_holder && C.admin_holder.rights & R_ADMIN) //Skip admins.
-				continue
-			if (C.is_afk(INACTIVITY_KICK))
-				if (!istype(C.mob, /mob/dead))
-					log_access("AFK: [key_name(C)]")
-					to_chat(C, SPAN_WARNING("You have been inactive for more than 10 minutes and have been disconnected."))
-					qdel(C)
+	if(list_clear_nulls(GLOB.clients))
+		debug_log("Removed nulls from GLOB.clients!")
+	if(list_clear_nulls(GLOB.player_list))
+		debug_log("Removed nulls from GLOB.player_list!")
+
+	if (!CONFIG_GET(flag/kick_inactive))
+		return
+
+	for(var/client/current as anything in GLOB.clients)
+		if(current.admin_holder && current.admin_holder.rights & R_ADMIN) //Skip admins.
+			continue
+		if(current.is_afk(INACTIVITY_KICK))
+			if(!istype(current.mob, /mob/dead))
+				log_access("AFK: [key_name(current)]")
+				to_chat(current, SPAN_WARNING("You have been inactive for more than [INACTIVITY_KICK / 600] minutes and have been disconnected."))
+				qdel(current)

code/controllers/subsystem/statpanel.dm

@@ -44,6 +44,9 @@ SUBSYSTEM_DEF(statpanels)
 		var/client/target = currentrun[length(currentrun)]
 		currentrun.len--
 
+		if(!target)
+			continue
+
 		if(!target.stat_panel.is_ready())
 			continue