becuase NULL isn't the same as ''
try
Code:
UPDATE xcart_products
SET descr = descr = '</p>' where descr = '' or descr is NULL;
however you may want to test the SQL does the right thing before committing such a change to actual data
so either run the update against a copy of your table or
Code:
select descr from xcart_products
where descr = '' or descr is NULL;