account_rule's get_invoice_line in sale.py doesn't like line types that are not 'line'

confirming a sale gives the following with non 'line' type lines with the account_rule module activated.

Traceback (most recent call last):
  File "/opt/trytond/trytond/protocols/dispatcher.py", line 204, in _dispatch
    result = rpc.result(meth(*c_args, **c_kwargs))
                        ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/trytond/trytond/model/modelview.py", line 720, in wrapper
    return func(cls, records, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/trytond/trytond/modules/sale/sale.py", line 1038, in process
    cls._process_invoice(sales)
  File "/opt/trytond/trytond/modules/sale/sale.py", line 1050, in _process_invoice
    invoice = sale.create_invoice()
              ^^^^^^^^^^^^^^^^^^^^^
  File "/opt/trytond/trytond/modules/sale_advance_payment/sale.py", line 380, in create_invoice
    invoice = super(Sale, self).create_invoice()
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/trytond/trytond/modules/sale/sale.py", line 870, in create_invoice
    invoice_lines.append(line.get_invoice_line())
                         ^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/trytond/trytond/modules/sale_secondary_unit/sale.py", line 155, in get_invoice_line
    lines = super().get_invoice_line()
            ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/trytond/trytond/modules/sale_option/sale.py", line 41, in get_invoice_line
    lines = super().get_invoice_line()
            ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/trytond/trytond/modules/sale_advance_payment/sale.py", line 449, in get_invoice_line
    lines = super(SaleLine, self).get_invoice_line()
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/trytond/trytond/modules/account_rule/sale.py", line 13, in get_invoice_line
    return_=self.quantity < 0):
            ^^^^^^^^^^^^^^^^^
TypeError: '<' not supported between instances of 'NoneType' and 'int'

don't see immediately the use of 'return_' here, but in any event it should probably have a guard such as return_=self.quantity < 0 if self.quantity else False