Add null check in `TestPlayer`'s disposal code to avoid cascading test failure

osu.Game/Tests/Visual/TestPlayer.cs

@@ -110,7 +110,7 @@ protected override void Dispose(bool isDisposing)
             // Specific to tests, the player can be disposed without OnExiting() ever being called.
             // We should make sure that the gameplay session has finished even in this case.
             if (LoadedBeatmapSuccessfully)
-                spectatorClient.EndPlaying(GameplayState);
+                spectatorClient?.EndPlaying(GameplayState);
         }
     }
 }