[MERGE] orm: add __iter__ method on browse_null.

Otherwise Python falls back to __getitem__ when trying to loop over it
or when trying to test `in`.

bzr revid: vmt@openerp.com-20130604081706-z79vqj36ls2adw09

openerp/osv/orm.py

@@ -282,6 +282,9 @@ 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