A Message from Planet XAML

<Label Content="(default)">
    <Label.Style>
        <Style TargetType="Label">
            <Style.Triggers>
                <DataTrigger Binding="{Binding Path=DefaultOutput, Converter={x:Static Converters:StringIsEmptyConverter.Instance}}" Value="False">
                    <Setter Property="Visibility" Value="Collapsed" />
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </Label.Style>
</Label>