Replace == null with is null

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -237,7 +237,7 @@ namespace SixLabors.ImageSharp.Formats.Png
                 }
 
                 // Use the metadata to determine what quantization depth to use if no quantizer has been set.
-                if (this.quantizer == null)
+                if (this.quantizer is null)
                 {
                     this.quantizer = new WuQuantizer(ImageMaths.GetColorCountForBitDepth(bits));
                 }

src/ImageSharp/MetaData/Profiles/Exif/ExifReader.cs

@@ -372,7 +372,7 @@ namespace SixLabors.ImageSharp.MetaData.Profiles.Exif
 
         private void AddInvalidTag(ExifTag tag)
         {
-            if (this.invalidTags == null)
+            if (this.invalidTags is null)
             {
                 this.invalidTags = new List<ExifTag>();
             }

src/ImageSharp/Processing/Processors/Dithering/PaletteDitherProcessorBase.cs

@@ -95,7 +95,7 @@ namespace SixLabors.ImageSharp.Processing.Processors.Dithering
             base.BeforeFrameApply(source, sourceRectangle, configuration);
 
             // Lazy init paletteVector:
-            if (this.paletteVector == null)
+            if (this.paletteVector is null)
             {
                 this.paletteVector = new Vector4[this.Palette.Length];
                 PixelOperations<TPixel>.Instance.ToScaledVector4(configuration, this.Palette, this.paletteVector);