Fix DockPanel index out of range (#18261)

* Update DockPanel.cs

* Update DockPanelTests.cs

src/Avalonia.Controls/DockPanel.cs

@@ -179,7 +179,7 @@ namespace Avalonia.Controls
                 }
             }
 
-            if (LastChildFill)
+            if (LastChildFill && Children.Count > 0)
             {
                 var child = Children[Children.Count - 1];
                 var childConstraint = new Size(
@@ -265,7 +265,7 @@ namespace Avalonia.Controls
                 }
             }
 
-            if (LastChildFill)
+            if (LastChildFill && Children.Count > 0)
             {
                 var child = Children[Children.Count - 1];
                 child.Arrange(new Rect(currentBounds.X, currentBounds.Y, currentBounds.Width, currentBounds.Height));

tests/Avalonia.Controls.UnitTests/DockPanelTests.cs

@@ -4,6 +4,21 @@ namespace Avalonia.Controls.UnitTests
 {
     public class DockPanelTests
     {
+        [Fact]
+        public void DockPanel_Without_Child()
+        {
+            var target = new DockPanel
+            {
+                Width = 10,
+                Height = 10
+            };
+
+            target.Measure(Size.Infinity);
+            target.Arrange(new Rect(target.DesiredSize));
+
+            Assert.Equal(new Rect(0, 0, 10, 10), target.Bounds);
+        }
+        
         [Fact]
         public void Should_Dock_Controls_Horizontal_First()
         {