Fix some nullrefs when running visualtests with no maps loaded.

osu.Game/Beatmaps/WorkingBeatmap.cs

@@ -30,6 +30,8 @@ namespace osu.Game.Beatmaps
                 {
                     if (background != null) return background;
 
+                    if (BeatmapInfo.Metadata?.BackgroundFile == null) return null;
+
                     try
                     {
                         using (var reader = GetReader())