add PinBox doc

doc/source/handycontrol/extend_controls/pinBox/index.md

@@ -1,3 +1,38 @@
 ---
-title: PinBox
+title: PinBox PIN码框
 ---
+
+密码框的另一种形式.
+
+```cs
+[TemplatePart(Name = ElementPanel, Type = typeof(Panel))]
+public class PinBox : Control
+```
+
+# 属性
+
+|属性|描述|默认值|备注|
+|-|-|-|-|
+|Password|密码||出于安全考虑，无法绑定|
+|PasswordChar|掩码字符|●||
+|Length|密码长度|4|最小值为4|
+|ItemMargin|单元框间隔|||
+|ItemWidth|单元框宽度|||
+|ItemHeight|单元框高度||||
+
+# 事件
+
+|名称|说明|
+|-|-|
+| Completed | 输入完成时触发 |
+
+# 案例
+
+```xml
+<StackPanel Margin="32" VerticalAlignment="Center" hc:PinBox.Completed="PinBox_OnCompleted">
+    <hc:PinBox Length="4" Password="1234"/>
+    <hc:PinBox Length="6" Password="123456" Margin="0,16,0,0" PasswordChar="❤"/>
+</StackPanel>
+```
+
+![PinBox](https://raw.githubusercontent.com/HandyOrg/HandyOrgResource/master/HandyControl/Resources/PinBox.png)

src/Shared/HandyControl_Shared/Controls/Input/PinBox.cs

@@ -175,10 +175,7 @@ namespace HandyControl.Controls
                 {
                     _passwordList = new List<SecureString>();
 
-                    if (value == null)
-                    {
-                        value = string.Empty;
-                    }
+                    value ??= string.Empty;
 
                     foreach (var item in value)
                     {