Fix nullref

the quantizier was initialized with default --> all properties are null. So we need to check that

src/ImageSharp/Formats/Gif/GifEncoderCore.cs

@@ -171,8 +171,11 @@ internal sealed class GifEncoderCore : IImageEncoderInternals
             quantized = null;
         }
 
+        if (hasPaletteQuantizer)
+        {
             paletteQuantizer.Dispose();
         }
+    }
 
     private void EncodeFrame<TPixel>(
         Stream stream,