fix(controls): Fix null binding error in scroll bar buttons when deleting tabs

[Koichi-Kobayashi]

Fixes NotSupportedException that occurred when deleting tabs. The issue was caused by RepeatButton.Content temporarily becoming null in scroll bar buttons, which failed to convert to SymbolRegular type via EnumConverter.
This issue was present in versions 4.0.3 through 4.1.0.

Pull request type

• Update
• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes

What is the current behavior?

The errors occurred in SymbolIcon elements for PART_ButtonScrollLeft and PART_ButtonScrollRight.

Issue Number: N/A

What is the new behavior?

• ScrollBar.xaml: Add TargetNullValue to UiScrollBarLineButton
• DynamicScrollBar.xaml: Add TargetNullValue to DynamicScrollBarLineButton

Other information

The following error occurs when deleting a tab.

例外がスローされました: 'System.NotSupportedException' (System.ComponentModel.TypeConverter.dll の中)
System.Windows.Data Error: 23 : Cannot convert '<null>' from type '<null>' to type 'Wpf.Ui.Controls.SymbolRegular' for 'en-US' culture with default conversions; consider using Converter property of Binding. NotSupportedException:'System.NotSupportedException: EnumConverter cannot convert from (null).
   at System.ComponentModel.TypeConverter.GetConvertFromException(Object value)
   at System.ComponentModel.TypeConverter.ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, Object value)
   at System.ComponentModel.EnumConverter.ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, Object value)
   at MS.Internal.Data.DefaultValueConverter.ConvertHelper(Object o, Type destinationType, DependencyObject targetElement, CultureInfo culture, Boolean isForward)'
例外がスローされました: 'System.NotSupportedException' (PresentationFramework.dll の中)
System.Windows.Data Error: 6 : 'ObjectSourceConverter' converter failed to convert value '<null>' (type '<null>'); fallback value will be used, if available. BindingExpression:Path=Content; DataItem='RepeatButton' (Name='PART_ButtonScrollLeft'); target element is 'SymbolIcon' (Name=''); target property is 'Symbol' (type 'SymbolRegular') NotSupportedException:'System.NotSupportedException: EnumConverter cannot convert from (null).
   at MS.Internal.Data.DefaultValueConverter.ConvertHelper(Object o, Type destinationType, DependencyObject targetElement, CultureInfo culture, Boolean isForward)
   at MS.Internal.Data.ObjectSourceConverter.Convert(Object o, Type type, Object parameter, CultureInfo culture)
   at System.Windows.Data.BindingExpression.ConvertHelper(IValueConverter converter, Object value, Type targetType, Object parameter, CultureInfo culture)'
例外がスローされました: 'System.NotSupportedException' (System.ComponentModel.TypeConverter.dll の中)
System.Windows.Data Error: 23 : Cannot convert '<null>' from type '<null>' to type 'Wpf.Ui.Controls.SymbolRegular' for 'en-US' culture with default conversions; consider using Converter property of Binding. NotSupportedException:'System.NotSupportedException: EnumConverter cannot convert from (null).
   at System.ComponentModel.TypeConverter.GetConvertFromException(Object value)
   at System.ComponentModel.TypeConverter.ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, Object value)
   at System.ComponentModel.EnumConverter.ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, Object value)
   at MS.Internal.Data.DefaultValueConverter.ConvertHelper(Object o, Type destinationType, DependencyObject targetElement, CultureInfo culture, Boolean isForward)'
例外がスローされました: 'System.NotSupportedException' (PresentationFramework.dll の中)
System.Windows.Data Error: 6 : 'ObjectSourceConverter' converter failed to convert value '<null>' (type '<null>'); fallback value will be used, if available. BindingExpression:Path=Content; DataItem='RepeatButton' (Name='PART_ButtonScrollRight'); target element is 'SymbolIcon' (Name=''); target property is 'Symbol' (type 'SymbolRegular') NotSupportedException:'System.NotSupportedException: EnumConverter cannot convert from (null).
   at MS.Internal.Data.DefaultValueConverter.ConvertHelper(Object o, Type destinationType, DependencyObject targetElement, CultureInfo culture, Boolean isForward)
   at MS.Internal.Data.ObjectSourceConverter.Convert(Object o, Type type, Object parameter, CultureInfo culture)
   at System.Windows.Data.BindingExpression.ConvertHelper(IValueConverter converter, Object value, Type targetType, Object parameter, CultureInfo culture)'