Add null-conditional to acesses in dispose methods

Such a terrible mistake, the finalizer may be called while the dependencies have an instance but the local itself doesn't have a value yet.

osu.Game/Rulesets/UI/DrawableRuleset.cs

@@ -343,7 +343,7 @@ namespace osu.Game.Rulesets.UI
             }
 
             // Dispose the components created by this dependency container.
-            dependencies.Dispose();
+            dependencies?.Dispose();
         }
     }