Fix build

Code analysis in rider did not catch that. only the build failed.

src/ImageSharp/Processing/Processors/Quantization/OctreeQuantizer{TPixel}.cs

@@ -26,7 +26,6 @@ public struct OctreeQuantizer<TPixel> : IQuantizer<TPixel>
     private readonly int maxColors;
     private readonly int bitDepth;
     private readonly Octree octree;
-    [NotNull]
     private IMemoryOwner<TPixel>? paletteOwner;
     private ReadOnlyMemory<TPixel> palette;
     private EuclideanPixelMap<TPixel>? pixelMap;
@@ -99,7 +98,7 @@ public struct OctreeQuantizer<TPixel> : IQuantizer<TPixel>
         }
 
         int paletteIndex = 0;
-        Span<TPixel> paletteSpan = this.paletteOwner.GetSpan();
+        Span<TPixel> paletteSpan = this.paletteOwner!.GetSpan();
 
         // On very rare occasions, (blur.png), the quantizer does not preserve a
         // transparent entry when palletizing the captured colors.
@@ -113,7 +112,7 @@ public struct OctreeQuantizer<TPixel> : IQuantizer<TPixel>
         }
 
         this.octree.Palletize(paletteSpan, max, ref paletteIndex);
-        ReadOnlyMemory<TPixel> result = this.paletteOwner.Memory[..paletteSpan.Length];
+        ReadOnlyMemory<TPixel> result = this.paletteOwner!.Memory[..paletteSpan.Length];
 
         // When called multiple times by QuantizerUtilities.BuildPalette
         // this prevents memory churn caused by reallocation.

src/ImageSharp/Processing/Processors/Quantization/PaletteQuantizer{TPixel}.cs

@@ -20,7 +20,6 @@ namespace SixLabors.ImageSharp.Processing.Processors.Quantization;
 internal struct PaletteQuantizer<TPixel> : IQuantizer<TPixel>
     where TPixel : unmanaged, IPixel<TPixel>
 {
-    [NotNull]
     private EuclideanPixelMap<TPixel>? pixelMap;
 
     /// <summary>
@@ -47,7 +46,7 @@ internal struct PaletteQuantizer<TPixel> : IQuantizer<TPixel>
     public QuantizerOptions Options { get; }
 
     /// <inheritdoc/>
-    public ReadOnlyMemory<TPixel> Palette => this.pixelMap.Palette;
+    public ReadOnlyMemory<TPixel> Palette => this.pixelMap!.Palette;
 
     /// <inheritdoc/>
     [MethodImpl(InliningOptions.ShortMethod)]
@@ -63,7 +62,7 @@ internal struct PaletteQuantizer<TPixel> : IQuantizer<TPixel>
     /// <inheritdoc/>
     [MethodImpl(InliningOptions.ShortMethod)]
     public readonly byte GetQuantizedColor(TPixel color, out TPixel match)
-        => (byte)this.pixelMap.GetClosestColor(color, out match);
+        => (byte)this.pixelMap!.GetClosestColor(color, out match);
 
     /// <inheritdoc/>
     public void Dispose()