Use null instead of UnsetValue for default property values

Author: robloo

Microsoft.Toolkit.Uwp.UI.Controls.Input/ColorPicker/ColorPicker.Properties.cs

@@ -20,7 +20,7 @@ public partial class ColorPicker
                 nameof(CustomPaletteColors),
                 typeof(ObservableCollection<Windows.UI.Color>),
                 typeof(ColorPicker),
-                new PropertyMetadata(DependencyProperty.UnsetValue));
+                new PropertyMetadata(null));
 
         /// <summary>
         /// Gets the list of custom palette colors.
@@ -64,7 +64,7 @@ public int CustomPaletteColumnCount
                 nameof(CustomPalette),
                 typeof(IColorPalette),
                 typeof(ColorPicker),
-                new PropertyMetadata(DependencyProperty.UnsetValue));
+                new PropertyMetadata(null));
 
         /// <summary>
         /// Gets or sets the custom color palette.