DB date field set to NULL

I have a form set up with a Date picker, the DB has a column with DATE type and a NULL value, the form save to a DB table the date value.
When I create a new record using my form nothing is shown in the date picker field , and this is what i need BUT if I submit the form with nothing in the date box, after the 'save to db' action, the DB field is filled with 0000-00-00 and not NULL as expected.
After that if I try to edit the record with my form a wrong date is shown in the date picker (it should be empty).

I have tried to add a custom action to set the value of the 'date' variable to NULL before saving into DB but I always get 0000-00-00 in the DB.....

Is there a way to save a NULL into my date field ?

Michele

Ok, my fault, the Custom action was set to Mode 'view' instead of 'Controller', after that the code below in the Custom action lead to a NULL in the saved DB field.

<?php 
if ($form->data['com_val'] == '') 
{
$form->data['com_val'] = NULL ;
}

if ($form->data['com_cliente'] == '') 
{
$form->data['com_cliente'] = NULL ;
}
?>

Michele