[FIX] sale_crm: wizard crm_make_sale crash if partner_id not sent on the opp.

Because defaults get function of partner_id of wizard read the partner_id of the opp and return the first item of the tuple, but if there isnt a partner on the opp, the read return a false for this field, not a tuple. 
No return the first item of the tuple if the partner_id is set, else False