Merge pull request #104 from peppy/fix-attrib-lookup-crash

Fix crash on looking up databased attributes when none are present

osu.Server.Queues.ScoreStatisticsProcessor/Stores/BeatmapStore.cs

@@ -76,7 +76,7 @@ public static async Task<BeatmapStore> CreateAsync(MySqlConnection? connection,
                     }, transaction: transaction)).ToArray();
             }
 
-            if (rawDifficultyAttributes == null)
+            if (rawDifficultyAttributes == null || rawDifficultyAttributes.Length == 0)
                 return null;
 
             DifficultyAttributes difficultyAttributes = LegacyRulesetHelper.CreateDifficultyAttributes(ruleset.RulesetInfo.OnlineID);