How to set ContentPresenter default content to Unset in WPF XAML - wpf

Question

I'm trying to display status icons in status bar. The icons are defined as ViewBox static resources and displayed via ContentPresenter style with DataTriggers.
I would like no icon displayed if none of the triggers are matched, so I've tried setting the default Setter Content to x:Null or an empty string or remove the line at all, but the other icons stop displaying at all then.
Any ideas please?
My XAML code is as follows
<StatusBarItem Grid.Column="2">
<ContentPresenter>
<ContentPresenter.Style>
<Style TargetType="ContentPresenter">
<Setter Property="Content" Value="{x:Null}"/>
<Style.Triggers>
<DataTrigger Binding="{Binding Path=State}" Value="Ok">
<Setter Property="Content" Value="{StaticResource StatusOK}"/>
</DataTrigger>
<DataTrigger Binding="{Binding Path=State}" Value="Invalid">
<Setter Property="Content" Value="{StaticResource StatusInvalid}"/>
</DataTrigger>
<DataTrigger Binding="{Binding Path=State}" Value="Warning">
<Setter Property="Content" Value="{StaticResource StatusWarning}"/>
</DataTrigger>
</Style.Triggers>
</Style>
</ContentPresenter.Style>
</ContentPresenter>
</StatusBarItem>
Update
I've tried using visibility as suggested by Ed Plunkett but the icons stopped showing up at all. Here is the code.
<Style TargetType="ContentPresenter">
<Setter Property="Content" Value="{StaticResource StatusOK}"/>
<Setter Property="Visibility" Value="Collapsed"/>
<Style.Triggers>
<DataTrigger Binding="{Binding Path=State}" Value="Ok">
<Setter Property="Content" Value="{StaticResource StatusOK}"/>
<Setter Property="Visibility" Value="Visible"/>
</DataTrigger>
<DataTrigger Binding="{Binding Path=State}" Value="Invalid">
<Setter Property="Content" Value="{StaticResource StatusInvalid}"/>
<Setter Property="Visibility" Value="Visible"/>
</DataTrigger>
<DataTrigger Binding="{Binding Path=State}" Value="Warning">
<Setter Property="Content" Value="{StaticResource StatusWarning}"/>
<Setter Property="Visibility" Value="Visible"/>
</DataTrigger>
</Style.Triggers>
</Style>

Answer 1

Your problem was misusing a ContentPresenter. There's no reason to use a ContentPresenter outside of a ControlTemplate. The only features it has which differ from ContentControl are things that only make sense inside a ControlTemplate. Now, as it happens, StatusBarItem is a subclass of ContentControl, so you could just style the StatusBarItem and set its Content. Either version in your question will work that way.