sale_credit_limit need to check used fields before using it
Following same concept from #12076 (closed). In a customized sale line type where has quantity but not unit price it fails with:
TypeError: unsupported operand type(s) for *: 'decimal.Decimal' and 'NoneType'
File "trytond/protocols/dispatcher.py", line 180, in _dispatch
result = rpc.result(meth(*c_args, **c_kwargs))
File "trytond/model/modelsql.py", line 871, in read
sub_ids, cls, field_list, values=sub_results)
File "trytond/model/fields/function.py", line 105, in get
return dict((name, call(name)) for name in names)
File "trytond/model/fields/function.py", line 105, in <genexpr>
return dict((name, call(name)) for name in names)
File "trytond/model/fields/function.py", line 98, in call
return method(records, name)
File "trytond/modules/dddddd/issue12076.py", line 45, in get_credit_amount
Decimal(str(quantity)) * line.unit_price, currency,