Apply NRT to DrawableOsuEditorRuleset too

osu.Game.Rulesets.Osu/Edit/DrawableOsuEditorRuleset.cs

@@ -1,8 +1,6 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-#nullable disable
-
 using System.Collections.Generic;
 using osu.Game.Beatmaps;
 using osu.Game.Rulesets.Mods;
@@ -25,7 +23,7 @@ namespace osu.Game.Rulesets.Osu.Edit
 
         private partial class OsuEditorPlayfield : OsuPlayfield
         {
-            protected override GameplayCursorContainer CreateCursor() => null;
+            protected override GameplayCursorContainer? CreateCursor() => null;
 
             public OsuEditorPlayfield()
             {