Original PR description

This error occurs when we create a new sales order, then proceed to create an
invoice within that sales order, and finally attempt to open the
``Accrued Revenue Entry`` from the action button.

Steps to reproduce:

- Install the ``sale_management`` and ``account_accountant`` modules
- Create new sales orders > ``Confirm`` > ``Create Invoice`` button >
  Down Payment in % > ``Confirm`` > ``Sale Order`` smart button
- Go to action button > ``Accrued Revenue Entry``

Traceback : 
``AssertionError precision_rounding must be positive, got 0.0``

At [1] within ``o.order_line``, we are encountering an issue where
``precision_rounding`` is being retrieved as zero. This is leading to an error.

This commit will fix the above error by implementing a check for the 
``is_downpayment`` in the order line.

[1]: https://github.com/odoo/odoo/blob/34b651de35b986db404a3950866bed7bc7e1fae2/addons/account/wizard/accrued_orders.py#L174

sentry-4857910663