Empty data and number format

Hello,

I load datas into a grid from a xml file.
Some elements of the xml file are numbers and sometimes they are empty.

They are loaded into a column that I format like this :
var format = new AW.Formats.Number;
format.setTextFormat("#.##");
obj.setCellFormat(format,4);

But if the element is empty, the text displayed in the grid is : 0.00

How can I force the column to display an empty string if the data is empty?