增加了Null2VisibilityConverter转换器

value是Null的时候隐藏，不为Null时显示

src/Shared/HandyControl_Shared/Tools/Converter/Null2VisibilityConverter.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Globalization;
+using System.Windows;
+using System.Windows.Data;
+
+namespace HandyControl.Tools.Converter
+{
+    public class Null2VisibilityConverter : IValueConverter
+    {
+        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            return value == null ? Visibility.Collapsed : Visibility.Visible;
+        }
+
+        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+        {
+            throw new NotImplementedException();
+        }
+    }
+}