Expand a Row in ListGrid cannot hide case

[wing.t.lee]

Refer to the sample in the showcase, and learn how to expand a row in ListGrid,

http://www.smartclient.com/smartgwt/...grid_memo_rows

From the example, it will show the "background" field when the row is expanded. I want to ask if I have real use case that some row have no "background" value, can I hide the arrow?
We want to make it use friend so that user don't need to click every arrow to check whether the "background" value here or not ...

        ListGrid listGrid = new ListGrid() {  
            @Override  
            protected Canvas getExpansionComponent(ListGridRecord record) {  
                Canvas canvas = super.getExpansionComponent(record);  
                canvas.setMargin(5);  
                return canvas;  
            }  
        };  
        listGrid.setWidth(600);  
        listGrid.setHeight(500);  
        listGrid.setCanExpandRecords(true);  
        listGrid.setExpansionMode(ExpansionMode.DETAIL_FIELD);  
        listGrid.setDetailField("background");  
  
        ListGridField nameField = new ListGridField("countryName", "Country");  
        ListGridField capitalField = new ListGridField("capital", "Capital");  
        ListGridField continentField = new ListGridField("continent", "Continent");  
        listGrid.setFields(nameField, capitalField, continentField);  
  
        listGrid.setData(CountrySampleData.getRecords());  
  
        listGrid.draw();