WPF Binding ValidationRules on TextBox - wpf

Question

This is my first attempt at using Binding.ValidationRules. They are working as expected with the exception of:
When I enter a TextBox value, for example: "abc" it's fine. When I backspace over letters "c", "b", then "a" the rule fires as it should and warns me the TextBox is empty. The strange behavior is that the string property, to which CompanyName is bound, still retains the "a". Therefore the string is not empty and I can't use the string.Empty comparison in my business logic.
It seems that the ValidationRule is preventing that last edit/character deletion to the string property.
I hope that makes sense, but I have included some images. The green "abc" Label in the image is bound to the same property as the TextBox.
What I want to do is control the visibility of a "Save" Button based on a valid TextBox. Pretty standard I would imagine, so I must be approaching this wrong or I a missing something simple.
<TextBox x:Name="CompanyNameTextBox" Grid.Row="1" TabIndex="1"
Style="{StaticResource TextBoxStyleWithError}"
Validation.ErrorTemplate="{StaticResource validationErrorTemplate}">
<TextBox.Text>
<Binding Path="objCompanyClass.CompanyName" UpdateSourceTrigger="PropertyChanged" >
<Binding.ValidationRules>
<HelperClass:CompanyNameValidator Max="50"/>
</Binding.ValidationRules>
</Binding>
</TextBox.Text>
</TextBox>
<Style x:Key="TextBoxStyleWithError" TargetType="TextBox">
<Style.Triggers>
<Trigger Property="Validation.HasError" Value="true">
<Setter Property="ToolTip" Value="{Binding RelativeSource={x:Static RelativeSource.Self}, Path=(Validation.Errors)[0].ErrorContent}"/>
<Setter Property="Background" Value="DarkRed"/>
<Setter Property="Margin" Value="3 15 3 3"/>
</Trigger>
</Style.Triggers>
</Style>
<ControlTemplate x:Key="validationErrorTemplate">
<DockPanel>
<StackPanel Orientation="Horizontal" DockPanel.Dock="Top" VerticalAlignment="Center" Margin="0 0 0 3">
<TextBlock Foreground="Red" FontSize="12" Margin="2,0,0,0"
Text="{Binding ElementName=ErrorAdorner, Path=AdornedElement.(Validation.Errors)[0].ErrorContent}">
</TextBlock>
</StackPanel>
<AdornedElementPlaceholder x:Name="ErrorAdorner" ></AdornedElementPlaceholder>
</DockPanel>
</ControlTemplate>
C#
public class CompanyNameValidator : ValidationRule
{
public static bool TextEntryValid = false;
private int _max;
public int Max
{
get { return _max; }
set { _max = value; }
}
public override ValidationResult Validate (object value, System.Globalization.CultureInfo cultureInfo)
{
if (string.IsNullOrEmpty(value.ToString()))
{
TextEntryValid = false;
return new ValidationResult(false, "Company Name cannot be empty.");
}
else
{
if (value.ToString().Length > Max)
{
TextEntryValid = false;
return new ValidationResult(false, $"Cannot be more than {Max} characters long.");
}
}
TextEntryValid = true;
return ValidationResult.ValidResult;
}
}