check if datetime field is null

How do I check if the field Original_Time is null?

$crud->callback_before_update(array($this,'check_Original_ETA'));



function check_Original_ETA($value, $primary_key)
{
$this -> db -> select('Original_ETA');
        $this -> db -> from('table');
        $this -> db -> where('id = ' . "'" . $primary_key . "'");
        $this -> db -> limit(1);
        $query = $this -> db -> get();
$row = $query->row();


$str = $row->Original_ETA;

if ($str != NULL)
{
   $crud->change_field_type('Original_ETA', 'readonly');
}
}

Code above is not working. Basically if field 'Original_Time' is already filled I don't want the user to be able to change this field again.

So I think I have two issues here. How to check for Null in DateTime and also how to set a field ReadOnly going to EDIT screen depending on a field value.

Thank you for helping.

Ron

Ron