[FIX] Add __iter__ method to browse_null to prevent server hang.

lp bug: https://launchpad.net/bugs/1172292 fixed

bzr revid: ronald@therp.nl-20130424142958-u5r2mxtxqj9v431h

openerp/osv/orm.py

@@ -285,6 +285,9 @@ class browse_null(object):
     def __unicode__(self):
         return u''
 
+    def __iter__(self):
+        raise NotImplementedError("Iteration is not allowed on %s" % self)
+
 
 #
 # TODO: execute an object method on browse_record_list