[FIX] [WIP] website_sale: fix crash when having product templates without variant.

probably it should not be displayed, but as there does not seem to exist any
clue about what the controller / view should do, I added some protections.

[DOC] added some comments because some methods seems quite weird.

bzr revid: tde@openerp.com-20131024110453-uj4xnrtwst0fdcrb