Skip null columns

In order to do this in the columns array...

```
'columns' => array(
        array(
            // .. 
        ),
        (isset($_REQUEST['showField1']) && $_REQUEST['showField1'])? array(
            'name'=>'field1',
            'value'=>'$data->field1',
        ) : null ,
        ...
     )

```

so you don't get this error...
```
Either "name" or "value" must be specified for CDataColumn.
```