fix polygon/polyline load fail points within the template：

Optimization fixed code

src/Avalonia.Controls/Shapes/Polygon.cs

@@ -1,18 +1,24 @@
 using System.Collections.Generic;
 using Avalonia.Media;
+using Avalonia.Data;
 
 namespace Avalonia.Controls.Shapes
 {
     public class Polygon : Shape
     {
         public static readonly StyledProperty<IList<Point>> PointsProperty =
-            AvaloniaProperty.Register<Polygon, IList<Point>>("Points", new Points());
+            AvaloniaProperty.Register<Polygon, IList<Point>>("Points");
 
         static Polygon()
         {
             AffectsGeometry<Polygon>(PointsProperty);
         }
 
+        public Polygon()
+        {
+            SetValue(PointsProperty, new Points(), BindingPriority.Template);
+        }
+
         public IList<Point> Points
         {
             get => GetValue(PointsProperty);

src/Avalonia.Controls/Shapes/Polyline.cs

@@ -1,5 +1,6 @@
 using System.Collections.Generic;
 using Avalonia.Media;
+using Avalonia.Data;
 
 namespace Avalonia.Controls.Shapes
 {
@@ -14,6 +15,11 @@ namespace Avalonia.Controls.Shapes
             AffectsGeometry<Polyline>(PointsProperty);
         }
 
+        public Polyline()
+        {
+            SetValue(PointsProperty, new Points(), BindingPriority.Template);
+        }
+
         public IList<Point> Points
         {
             get => GetValue(PointsProperty);