public string NullValueLabel {get; set;}

string colorBrewerSchemesName = "ArcGIS Colors";
StyleProjectItem style = Project.Current.GetItems<StyleProjectItem>().First(s => s.Name == colorBrewerSchemesName);
string colorRampName = "Heat Map 4 - Semitransparent";
IList<ColorRampStyleItem> colorRampList = await QueuedTask.Run(() =>
{
  return style.SearchColorRamps(colorRampName);
});
ColorRampStyleItem colorRamp = colorRampList[0];

await QueuedTask.Run(() =>
{
  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.ColorRamp, "Highest", "Lowest", 5000000, 50000)
  {

    //drawing features with null values with a different symbol
    ShowNullValues = true,
    NullValueLabel = "Unknown"
  };
  CIMPointSymbol nullSym = SymbolFactory.Instance.ConstructPointSymbol(ColorFactory.Instance.RedRGB, 16.0, SimpleMarkerStyle.Circle);
  unclassRndrDef.NullValueSymbol = nullSym.MakeSymbolReference();
  FeatureLayer flyr = MapView.Active.Map.Layers[0] as FeatureLayer;
  CIMClassBreaksRenderer cbRndr = flyr.CreateRenderer(unclassRndrDef) as CIMClassBreaksRenderer;
  flyr.SetRenderer(cbRndr);
});