Don't die on a null connection during disposal

May be caused by a startup error. This shouldn't result in an additional exception here.

osu.Game/OsuGameBase.cs

@@ -231,7 +231,7 @@ namespace osu.Game
                 LocalConfig.Save();
             }
 
-            connection.Dispose();
+            connection?.Dispose();
 
             base.Dispose(isDisposing);
         }