Port LP MP https://code.launchpad.net/~camptocamp/openerp-rma/rma-fix-lp1311520_rde/+merge/216839

it fixes partially lp:~camptocamp/openerp-rma/rma-fix-lp1311520_rde

Invoice lines without product_id are possible.
We need to manage this case, for example it seems at least relevant to have the possibility to generate a refund.

About actions "new products return" and "new delivery", is there really a functional need when there is no product_id on invoice line?