增加Null2VisibilityConverter

value是null的时候隐藏，不为null显示

HandyControl/HandyControl.csproj

@@ -156,6 +156,7 @@
     <Compile Include="Data\Gif\GPStream.cs" />
     <Compile Include="Tools\Converter\Boolean2StrConverter.cs" />
     <Compile Include="Tools\Converter\Int2StrConverter.cs" />
+    <Compile Include="Tools\Converter\Null2VisibilityConverter.cs" />
     <Compile Include="Tools\Helper\ColorHelper.cs" />
     <Compile Include="Tools\GifImageAnimator.cs" />
     <Compile Include="Data\GrowlInfo.cs" />

HandyControl/Themes/Basic/Converters.xaml

@@ -15,5 +15,6 @@
     <converter:Object2BooleanConverter x:Key="Object2BooleanConverter"/>
     <converter:Boolean2StrConverter x:Key="Boolean2StrConverter"/>
     <converter:Int2StrConverter x:Key="Int2StrConverter"/>
+    <converter:Null2VisibilityConverter x:Key="Null2VisibilityConverter"/>
 
 </ResourceDictionary>

HandyControl/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();
+        }
+    }
+}