Magento : Filter Collection by Attribute that Doesn't Exist or Null or another condition

After some tests, the code I wrote in the third edit of the question seems to work fine, so I think it is the correct answer:

$_products = Mage::getModel('catalog/product')
  ->getCollection()
  ->addAttributeToSelect(array('sku', 'name', 'newfield'))
  ->addAttributeToFilter(
    array(
        array('attribute'=> 'newfield','null' => true),
        array('attribute'=> 'newfield','eq' => ''),
        array('attribute'=> 'newfield','eq' => 'NO FIELD')
    ),
    '',
    'left');

I checked it and it returns me the products where the newfield is not set yet, and the products where newfield is NULL, string(0) or string(8) "NO FIELD".

PD: if only need one condition and do not need the OR, then it is enough with:

$_products = Mage::getModel('catalog/product')
    ->getCollection()
    ->addAttributeToSelect(array('sku', 'name', 'newfield'))
    ->addAttributeToFilter('newfield', array('null' => true), 'left');