[FIX]Improved field int res_user_view.xml tree view
[FIX] Add __iter__ method to browse_null to prevent server hang.
[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`.
[MERGE] res_users treee view: use the `login_date` field (instead of `date`).