[FIX] portal_sale: force empty context

The context was removed by 1933e926.

addons/portal_sale/portal_sale_view.xml

@@ -64,6 +64,7 @@
             <field name="res_model">account.invoice</field>
             <field name="view_mode">tree,form</field>
             <field name="domain">[('type','in',['out_invoice','out_refund'])]</field>
+            <field name="context">{}</field>
             <field name="search_view_id" ref="account.view_account_invoice_filter"/>
             <field name="help">We haven't sent you any invoice.</field>
         </record>