Set StarDifficulty to -1 for invalid beatmaps

The StarDifficulty calculation in a BeatmapGroup would not properly deal
with Beatmaps that cannot be loaded and thus cause a
NullReferenceException when trying to work with them, instead of
assigning a StarDifficulty of -1 to signify that the map is invalid.

osu.Game/Beatmaps/Drawables/BeatmapGroup.cs

@@ -64,7 +64,7 @@ namespace osu.Game.Beatmaps.Drawables
             BeatmapSet = beatmapSet;
             WorkingBeatmap beatmap = database.GetWorkingBeatmap(BeatmapSet.Beatmaps.FirstOrDefault());
             foreach (var b in BeatmapSet.Beatmaps)
-                b.StarDifficulty = (float)database.GetWorkingBeatmap(b).Beatmap.CalculateStarDifficulty();
+                b.StarDifficulty = (float)(database.GetWorkingBeatmap(b).Beatmap?.CalculateStarDifficulty() ?? -1f);
 
             Header = new BeatmapSetHeader(beatmap)
             {