chore: complete avalonia button style.

src/Shared/HandyControl_Shared/Tools/Converter/BorderCircularConverter.cs

@@ -9,18 +9,18 @@ public class BorderCircularConverter : IMultiValueConverter
 {
     public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
     {
-        if (values.Length == 2 && values[0] is double width && values[1] is double height)
+        if (values.Length != 2 || values[0] is not double width || values[1] is not double height)
         {
-            if (width < double.Epsilon || height < double.Epsilon)
-            {
-                return new CornerRadius();
-            }
-
-            var min = Math.Min(width, height);
-            return new CornerRadius(min / 2);
+            return DependencyProperty.UnsetValue;
         }
 
-        return DependencyProperty.UnsetValue;
+        if (width < double.Epsilon || height < double.Epsilon)
+        {
+            return new CornerRadius();
+        }
+
+        var min = Math.Min(width, height);
+        return new CornerRadius(min / 2);
     }
 
     public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)