Do not cache null values in beatmap lookup cache

[bdach]

RFC.

The reason I'm bringing this up is #37383.

In this case, the retrieval of the beatmap failed on a timeout:

2026-04-19 02:43:10 [verbose]: Request to https://osu.ppy.sh/api/v2/beatmaps/?ids[]=5090069 failed with System.Net.WebException: Request to https://osu.ppy.sh/api/v2/beatmaps/?ids[]=5090069 timed out after 10 seconds idle (read 0 bytes, retried 0 times)..
2026-04-19 02:43:10 [verbose]: Failing request osu.Game.Online.API.Requests.GetBeatmapsRequest (System.Net.WebException: Request to https://osu.ppy.sh/api/v2/beatmaps/?ids[]=5090069 timed out after 10 seconds idle (read 0 bytes, retried 0 times).)

2026-04-19 02:43:10 [error]: Failed to load beatmap 5090069 for playlistItem 0.
2026-04-19 02:43:10 [verbose]: ⚠️ Failed to load beatmap 5090069 for playlistItem 0.

This fails at

osu/osu.Game/Screens/OnlinePlay/Matchmaking/RankedPlay/Card/RankedPlayCard.cs

Lines 167 to 171 in 0e9664b

	if (beatmap == null)
	{
	Logger.Log($"Failed to load beatmap {playlistItem.BeatmapID} for playlistItem {playlistItem.ID}.", level: LogLevel.Error);
	return;
	}

Just this failing wouldn't cause the game to soft-lock; when I suppress the lookup locally, the game continues as normal. However, suppressing the lookup is not the same as the lookup failing, because a failed lookup will write a null to the cache, which means that when a beatmap download is initiated later in

osu/osu.Game/Screens/OnlinePlay/Matchmaking/Match/ScreenMatchmaking.cs

Lines 308 to 320 in 1b48894

	// In a perfect world we'd use BeatmapAvailability, but there's no event-driven flow for when a selection changes.
	// ie. if selection changes from "not downloaded" to another "not downloaded" we wouldn't get a value changed raised.
	beatmapLookupCache
	.GetBeatmapAsync(item.BeatmapID, (downloadCheckCancellation = new CancellationTokenSource()).Token)
	.ContinueWith(resolved => Schedule(() =>
	{
	APIBeatmapSet? beatmapSet = resolved.GetResultSafely()?.BeatmapSet;
	if (beatmapSet == null)
	return;
	beatmapDownloader.Download(beatmapSet, config.Get<bool>(OsuSetting.PreferNoVideo));
	}));

it'll just do nothing and the user will be stuck, even though they could very well attempt to download the beatmap here if the lookup were to succeed on the second go.

To a degree changing the whole cache for this could be viewed as the tail wagging the dog, but I think in general caching nulls here seems pretty anti-user. From within the client I would generally not expect very many complete misses when looking up beatmaps.

Note that this wouldn't soft-lock the match anymore since ppy/osu-server-spectator#471. It's just another step in making degradation less impactful.