Added WinUI project.

Harmonia.WinUI/Converters/NullVisibilityConverter.cs

@@ -0,0 +1,27 @@
+using Microsoft.UI.Xaml.Data;
+using Microsoft.UI.Xaml;
+using System;
+using System.Collections;
+
+namespace Harmonia.WinUI.Converters;
+
+public sealed partial class NullVisibilityConverter : IValueConverter
+{
+    public NullVisibilityConverter()
+    {
+
+    }
+
+    public object Convert(object value, Type targetType, object parameter, string language)
+    {
+        if (value is not IList list)
+            return Visibility.Collapsed;
+        
+        return list.Count == 0 ? Visibility.Collapsed : Visibility.Visible;
+    }
+
+    public object ConvertBack(object value, Type targetType, object parameter, string language)
+    {
+        throw new NotImplementedException();
+    }
+}