无法在Extjs中的网格内呈现空(Null)列

// 定义列模型
var columnModel = new Ext.grid.ColumnModel([
    {header: '姓名', dataIndex: 'name'},
    {header: '年龄', dataIndex: 'age', renderer: function(value) {
        if (value === null || value === undefined) {
            return '空';
        }
        return value;
    }},
    {header: '性别', dataIndex: 'gender'}
]);

// 定义数据源
var store = new Ext.data.Store({
    fields: ['name', 'age', 'gender'],
    data: [
        {name: '张三', age: 20, gender: '男'},
        {name: '李四', age: null, gender: '女'},
        {name: '王五', age: 30, gender: '男'}
    ]
});

// 创建网格控件
var grid = new Ext.grid.GridPanel({
    store: store,
    cm: columnModel,
    renderTo: 'grid-container'
});