title: How to Set a Value to NULL when using Zend_Db
body: <p>When performing UPDATE and INSERT queries using Zend_Db, I frequently need to set values equal to NULL (not ''). However, the default behavior of Zend_Db::insert() and Zend_Db::update() seems to be that values that are empty are translated into empty strings ('') and put into the database as such. </p>
  
  <p>Does anyone know of way to actually force a NULL value to go into fields if the value is empty in php?</p>