public string NullValueLabel {get; set;}

{
    string colorBrewerSchemesName = "ArcGIS Colors";
    //Get the style project item that contains the color ramps
    //Refer to the Initialize region for an example of how to get a style item
    //Note: Run within a QueuedTask
    CIMPointSymbol pointSym = SymbolFactory.Instance.ConstructPointSymbol(ColorFactory.Instance.GreenRGB, 16.0, SimpleMarkerStyle.Diamond);
    CIMSymbolReference symbolPointTemplate = pointSym.MakeSymbolReference();

    //defining an unclassed renderer with custom upper and lower stops
    //all features with value >= 5,000,000 will be drawn with the upper color from the color ramp
    //all features with value <= 50,000 will be drawn with the lower color from the color ramp
    UnclassedColorsRendererDefinition unclassRndrDef = new UnclassedColorsRendererDefinition
                          ("Population", symbolPointTemplate, colorRamp, "Highest", "Lowest", 5000000, 50000)
    {

        //drawing features with null values with a different symbol
        ShowNullValues = true,
        NullValueLabel = "Unknown"
    };
    // Create a point symbol for null values
    CIMPointSymbol nullSym = SymbolFactory.Instance.ConstructPointSymbol(ColorFactory.Instance.RedRGB, 16.0, SimpleMarkerStyle.Circle);
    unclassRndrDef.NullValueSymbol = nullSym.MakeSymbolReference();
    //Create the unclassed renderer using the definition
    CIMClassBreaksRenderer cbRndr = featureLayer.CreateRenderer(unclassRndrDef) as CIMClassBreaksRenderer;
    //Set the renderer to the feature layer
    featureLayer.SetRenderer(cbRndr);
}