Add :empty pseudoclass for ContentPresenter

src/Avalonia.Controls/Presenters/ContentPresenter.cs

@@ -1,9 +1,9 @@
 using System;
+
+using Avalonia.Controls.Metadata;
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Templates;
 using Avalonia.Controls.Utils;
-using Avalonia.Data;
-using Avalonia.Input;
 using Avalonia.Layout;
 using Avalonia.LogicalTree;
 using Avalonia.Media;
@@ -14,6 +14,7 @@ namespace Avalonia.Controls.Presenters
     /// <summary>
     /// Presents a single item of data inside a <see cref="TemplatedControl"/> template.
     /// </summary>
+    [PseudoClasses(":empty")]
     public class ContentPresenter : Control, IContentPresenter
     {
         /// <summary>
@@ -102,6 +103,11 @@ namespace Avalonia.Controls.Presenters
             TemplatedParentProperty.Changed.AddClassHandler<ContentPresenter>((x, e) => x.TemplatedParentChanged(e));
         }
 
+        public ContentPresenter()
+        {
+            UpdatePseudoClasses();
+        }
+
         /// <summary>
         /// Gets or sets a brush with which to paint the background.
         /// </summary>
@@ -424,9 +430,15 @@ namespace Avalonia.Controls.Presenters
                 _recyclingDataTemplate = null;
             }
 
+            UpdatePseudoClasses();
             InvalidateMeasure();
         }
 
+        private void UpdatePseudoClasses()
+        {
+            PseudoClasses.Set(":empty", Content is null);
+        }
+
         private double GetLayoutScale()
         {
             var result = (VisualRoot as ILayoutRoot)?.LayoutScaling ?? 1.0;