Change difficulty cache storage type to nullable

The recent changes related to adding support for working beatmap load
cancellation exposed a flaw in the beatmap difficulty cache. With the
way the difficulty computation logic was written, any error in the
calculation process (including beatmap load timeout, or cancellation)
would result in a 0.00 star rating being permanently cached in memory
for the given beatmap.

To resolve, change the difficulty cache's return type to nullable.
In failure scenarios, `null` is returned, rather than
`default(StarDifficulty)` as done previously.

osu.Game.Tests/Beatmaps/TestSceneBeatmapDifficultyCache.cs

@@ -164,9 +164,9 @@ namespace osu.Game.Tests.Beatmaps
         {
             public Func<DifficultyCacheLookup, StarDifficulty> ComputeDifficulty { get; set; }
 
-            protected override Task<StarDifficulty> ComputeValueAsync(DifficultyCacheLookup lookup, CancellationToken token = default)
+            protected override Task<StarDifficulty?> ComputeValueAsync(DifficultyCacheLookup lookup, CancellationToken token = default)
             {
-                return Task.FromResult(ComputeDifficulty?.Invoke(lookup) ?? new StarDifficulty(BASE_STARS, 0));
+                return Task.FromResult<StarDifficulty?>(ComputeDifficulty?.Invoke(lookup) ?? new StarDifficulty(BASE_STARS, 0));
             }
         }
     }