Don't cache invalid/missing difficulty attributes

[smoogipoo]

Sorry this took so long... Tests were hard to make.

As mentioned in #282 (comment), and has now popped up in practice: https://discord.com/channels/188630481301012481/380598781432823815/1300484977992929443 .

Errors will be something like...

osu.Server.Queues.ScoreStatisticsProcessor.Stores.DifficultyAttributesMissingException: DifficultyAttributeKey { BeatmapId = 1, RulesetId = 0, ModValue = 0 }
 ---> System.Collections.Generic.KeyNotFoundException: The given key '9' was not present in the dictionary.
   at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
   at osu.Game.Rulesets.Difficulty.DifficultyAttributes.FromDatabaseAttributes(IReadOnlyDictionary`2 values, IBeatmapOnlineInfo onlineInfo)