When there are no data points in the bar series, the chart fails to retrieve a color for the marker of the legend item. This can be worked around by modifying the item template of the legend item. In the new template the binding of the Fill of the marker, should include not just the MarkerFill but TargetNullValue:

<DataTemplate x:Key="legendItemContentTemplate1">
 <StackPanel Orientation="Horizontal">
  <Path Fill="{Binding MarkerFill, TargetNullValue=#9B25B1}" Width="12" Height="12">
   <Path.Data>
    <Binding Path="ActualMarkerGeometry" 
      RelativeSource="{RelativeSource AncestorType=telerik:LegendItemControl}" />
   </Path.Data>
  </Path>
  <TextBlock Margin="2" Text="{Binding Title}" />
 </StackPanel>
</DataTemplate>


Will be available in Q2 2016 Release.