Nullable annotate DrawableSample

osu.Framework/Graphics/Audio/DrawableSample.cs

···
 // Copyright (c) ppy Pty Ltd <[e-mail]>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
+#nullable enable
+
 using osu.Framework.Audio.Mixing;
 using osu.Framework.Audio.Sample;
 using osu.Framework.Bindables;
···
 
         public Bindable<int> PlaybackConcurrency { get; } = new Bindable<int>(Sample.DEFAULT_CONCURRENCY);
 
-        private IAudioMixer mixer;
+        private IAudioMixer? mixer;
 
         protected override void OnMixerChanged(ValueChangedEvent<IAudioMixer> mixer)
         {