Declare a grid resource to be used as datasource similar to the following
<Grid x:Key="ItemsData">
<Grid>
<Grid.Resources>
<sys:String x:Key="Name">AA</sys:String>
</Grid.Resources>
</Grid>
<Grid>
<Grid.Resources>
<sys:String x:Key="Name">BB</sys:String>
<sys:Double x:Key="DurabilityPercentage">66</sys:Double>
</Grid.Resources>
</Grid>
</Grid>

Use that data source as ItemsSource in a ListBox
<ListBox ItemsSource="{Binding Source={StaticResource ItemsData}, Path=Children}"
Style="{StaticResource ListboxStyle}"
SelectedIndex="0"/>

Ensure ListBoxStyle Binds data context to the Resources of each child item
<Setter Property="ItemTemplate">
<Setter.Value>
<DataTemplate>
<ContentControl DataContext="{Binding Resources}"
Style="{StaticResource Item}" />
</DataTemplate>
</Setter.Value>
</Setter>

Make a trigger on Item template similar to the following
<Grid x:Name="ItemFrame">
<ProgressBar x:Name="DurabilityBar"
Value="{Binding [DurabilityPercentage]}"/>
</Grid>
<ControlTemplate.Triggers>
<DataTrigger Binding="{Binding [DurabilityPercentage]}" Value="{x:Null}">
<Setter TargetName="DurabilityBar" Property="Visibility" Value="Collapsed" />
</DataTrigger>
</ControlTemplate.Triggers>

Expected: The items with no DurabilityPercentage, are Collapsed
Actual: The items with no DurabilityPercentage, are Visible