Fixes rotation breaking on one votable map (#96493)

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## About The Pull Request

Basically, if it's forced, we rely on create_vote to catch the "no
options" case.
This used to be the behavior, but it was removed in persistant tallies.
I have no idea why.
I've also cleaned up the quite dubious caching code to avoid the bugs I
was getting when testing locally.

## Why It's Good For The Game

S been causing forced icebox (15 pop min, meta can't replace it because
meta's the only one with 0 minpop) on the servers, that's silly.

Of note, it's possible this will still be an issue due to the stale map
system, but yaknow, one thing at a time, and that'll be louder so we
should find out pretty fast (fix would be defaulting to some marked
default map if only one can be rolled).

## Changelog

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🆑
fix: End of round mapvotes will now properly default to the only option
if only one map is votable.
/🆑

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code/controllers/subsystem/map_vote.dm

@@ -22,6 +22,9 @@ SUBSYSTEM_DEF(map_vote)
 	/// Stores the last amount of potential players to compare next time we're called
 	var/player_cache = -1
 
+	/// Cached list of votable maps
+	var/list/votable_map_cache
+
 	/// Stores a formatted html string of the tally counts
 	var/tally_printout = span_red("Loading...")
 
@@ -113,8 +116,9 @@ SUBSYSTEM_DEF(map_vote)
 	else
 		filter_threshold = length(GLOB.clients)
 
-	if(filter_threshold == player_cache)
-		return null
+	// Cached because it's called off ui_data, I really don't think this is worth it but whatever
+	if(filter_threshold == player_cache && !isnull(votable_map_cache))
+		return votable_map_cache.Copy()
 
 	player_cache = filter_threshold
 	var/list/valid_maps = list()
@@ -130,6 +134,7 @@ SUBSYSTEM_DEF(map_vote)
 			continue
 		valid_maps += possible_config.map_name
 
+	votable_map_cache = valid_maps.Copy()
 	return valid_maps
 
 /datum/controller/subsystem/map_vote/proc/filter_cache_to_valid_maps()