[FIX] purchase: Purchase Analysis view was using incorrect JOIN order

[FIX] purchase: Purchase Analysis view was using incorrect JOIN order

The starting table for Purchase Analysis is purchase_order_line
not purchase_order. The previous code was using a wrong JOIN
combination starting from purchase_order, which resulted in
a crash if an empty PO was created.
In order to prevent this an extra  WHERE clause on product_id
being NOT NULL was added, but this was incorrect too as it
prevented PO lines with no product_id value from appearing
in the Purchase Analysis results, while being perfectly valid.

bzr revid: odo@openerp.com-20131104165826-kltuzlh4i8q89sk0