Fix null Name on empty page header description (MAS 4.1.2) (#781)

The PageHeader description Label is a focusable tab stop, but its
visibility used NullToVisibilityConverter, which only collapses on null -
not on an empty string. Pages whose description is "" (Label, TextBox,
TextBlock, RichTextEdit, PasswordBox and many others) therefore rendered
an empty, focusable element with a null UI Automation Name, flagged by
Accessibility Insights as "The Name property of a focusable element must
not be null" (MAS 4.1.2 - Name, Role, Value).

- Switch the description Label visibility to EmptyToVisibilityConverter so
  it collapses on null, empty, or whitespace, removing the empty focusable
  element entirely.
- Add AutomationProperties.Name bound to Description so the description has
  a meaningful Name when it is visible.
- Remove the now-unused NullToVisibilityConverter resource declaration.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: amarinov-msft

Sample Applications/WPFGallery/Controls/PageHeader.xaml

@@ -4,7 +4,7 @@
     xmlns:controls="clr-namespace:WPFGallery.Controls"
     xmlns:helpers="clr-namespace:WPFGallery.Helpers">
 
-    <helpers:NullToVisibilityConverter x:Key="NullToVisibilityConverter"/>
+    <helpers:EmptyToVisibilityConverter x:Key="EmptyToVisibilityConverter"/>
 
     <Style TargetType="{x:Type controls:PageHeader}">
         <Setter Property="Focusable" Value="False"/>
@@ -25,10 +25,11 @@
                         </Label>
 
                         <Label
+                            AutomationProperties.Name="{Binding Description, RelativeSource={RelativeSource Mode=TemplatedParent}}"
                             AutomationProperties.HeadingLevel="Level2"
                             KeyboardNavigation.IsTabStop="True"
                             KeyboardNavigation.TabIndex="1"
-                            Visibility="{TemplateBinding Description, Converter={StaticResource NullToVisibilityConverter}}"
+                            Visibility="{TemplateBinding Description, Converter={StaticResource EmptyToVisibilityConverter}}"
                             Focusable="True">
                             <TextBlock
                                 x:Name="DescriptionTextBlock"