Fix crash when toggling extended statistics visibility during results…

… load

Closes #29066.

Initially I fixed this at where the assert is right now:

	https://github.com/ppy/osu/blob/9790c5a574b782c41c8c6da99ad8c42dfadc9de8/osu.Game/Screens/Ranking/ResultsScreen.cs#L333

but because of the weird way that visible state management is done in
this screen that made it possible for the extended statistics to be
visible *behind* the score panels, without the score panels making way
for it. So this is in a way safer, because it prevents the visibility
state of the extended statistics from changing in the first place if
there is no score selected (yet).

This can be also seen in playlists, at least.

osu.Game/Screens/Ranking/ResultsScreen.cs

@@ -398,7 +398,8 @@ public bool OnPressed(KeyBindingPressEvent<GlobalAction> e)
                     break;
 
                 case GlobalAction.Select:
-                    StatisticsPanel.ToggleVisibility();
+                    if (SelectedScore.Value != null)
+                        StatisticsPanel.ToggleVisibility();
                     return true;
             }