How to make YUI datasource parse Null values in the dataset

var columnDescription =
		    [
				{key:'Requirements'},
				{key:'abc'},
				{key:'xyz'}
		     ];

var dataSrcSample = new YAHOO.util.FunctionDataSource(getDataGrid);
  myDataSource.connMethodPost = true;
  myDataSource.responseType = YAHOO.util.DataSource.TYPE_JSON;
                	myDataSource.responseSchema = {
                   	 fields:['Requirements',
    {key:'abc',parser:YAHOO.util.DataSource.parseString},
     {key:'xyz',parser:YAHOO.util.DataSource.parseString}]
                    	};

YAHOO.example.sampleTable = function()
		{
                           var columnDesc=columnDescription;
			var myDataSource = dataSrcSample;
			var oConfigs =
			{
				width:'100%'
			};

var myDataTable = new YAHOO.widget.DataTable("tableContainerDiv",
    columnDesc,
     myDataSource,
     oConfigs);
        		}();

function getDataGrid()
{
      //calls backend and gets the data
}

function parseNull(value) {
    // This exact logic may be incorrect, depends on what you get for value in the null case
    if (value=='null') {
        return null;
    }
    YAHOO.util.DataSource.parseString(value);
}

{key:'abc',parser:parseNull}