EmptyComboBoxStyle.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:local="clr-namespace:BVCareManager"
                    xmlns:materialDesign="http://materialdesigninxaml.net/winfx/xaml/themes">


    <Style TargetType="ComboBox" 
           BasedOn="{StaticResource MaterialDesignFloatingHintComboBox}" 
           x:Key="EmptyComboBoxStyle">

        <Style.Triggers>
            <MultiTrigger>
                <MultiTrigger.Conditions>
                    <Condition Property="SelectedItem" Value="{x:Null}"/>
                </MultiTrigger.Conditions>
                <Setter Property="BorderBrush" Value="Red"/>
            </MultiTrigger>

            <MultiTrigger>
                <MultiTrigger.Conditions>
                    <Condition Property="SelectedItem" Value=""/>
                </MultiTrigger.Conditions>
                <Setter Property="BorderBrush" Value="Red"/>
            </MultiTrigger>
        </Style.Triggers>
    </Style>
    
</ResourceDictionary>