Fix IsValueCreated method not cecking whether the async task was completed

Caused potential stutters for components that relied on this check.

osu.Game/Beatmaps/WorkingBeatmap.cs

@@ -144,7 +144,7 @@ namespace osu.Game.Beatmaps
                 get
                 {
                     ensureValid();
-                    return lazy.IsValueCreated;
+                    return lazy.Value.IsCompleted;
                 }
             }