before this commit, traceback is raised when trying to create accrued revenue entry without entering the order lines.

open a sale order, enter customer and save it, from the action button, click Accrued Revenue Entry and enter an amount in the amount field, exception will be raised.

after this commit, no exception will be raised.

closes odoo/odoo#110308

Signed-off-by: John Laterre (jol) <jol@odoo.com>