Fix TreeView Expander Visibility

[StefanoRivolta-Previero]

When a TreeViewItem doesn't have any children items inside, the expander should be collapsed, otherwhise it creates a ghost minimum height that can't be removed.

Example:

given this treeview:

<TreeView
    Grid.Row="0"
    ItemsSource="{Binding MovieCategories}"
    MinWidth="220" 
    SelectedItemChanged="TreeView_SelectedItemChanged">
    <TreeView.Resources>
        <Style TargetType="TreeViewItem" BasedOn="{StaticResource MaterialDesignTreeViewItem}">
            <Setter Property="materialDesign:TreeViewAssist.ExpanderSize" Value="32"/>
            <Setter Property="materialDesign:TreeViewAssist.ShowSelection" Value="False"/>
            <Setter Property="Background" Value="Green"/>
            <Setter Property="Padding" Value="0"/>
        </Style>
        <HierarchicalDataTemplate
            DataType="{x:Type domain:MovieCategory}"
            ItemsSource="{Binding Movies}">
            <TextBlock 
                Text="{Binding Name}"
                Margin="10"/>
        </HierarchicalDataTemplate>

        <DataTemplate DataType="{x:Type domain:Movie}">
            <TextBlock 
                Text="{Binding Name}"
                Background="red"
                Margin="8"
                ToolTip="{Binding Director}"/>
        </DataTemplate>
    </TreeView.Resources>
</TreeView>

When setting the textblock visibility to collapsed

Before

After