Crash on production cost when purchase line have no amount
As unit price is not enforced on purchase line, `line.amount` can be None, producing the following crash on computing the production cost of the outsourcing. ``` Traceback (most recent call last): File "/trytond/wsgi.py", line 109, in dispatch_request return endpoint(request, **request.view_args) File "/trytond/protocols/dispatcher.py", line 43, in rpc return methods.get(request.rpc_method, _dispatch)( File "/trytond/wsgi.py", line 75, in wrapper return func(request, *args, **kwargs) File "/trytond/protocols/wrappers.py", line 197, in wrapper return func(request, pool, *args, **kwargs) File "/trytond/protocols/dispatcher.py", line 196, in _dispatch result = rpc.result(meth(*c_args, **c_kwargs)) File "/trytond/model/modelsql.py", line 1171, in read getter_results = field.get( File "/trytond/model/fields/function.py", line 27, in wrapper return func(self, *args, **kwargs) File "/trytond/transaction.py", line 50, in wrapper return func(*args, **kwargs) File "/trytond/model/fields/function.py", line 159, in get return dict((name, call(name)) for name in names) File "/trytond/model/fields/function.py", line 159, in <genexpr> return dict((name, call(name)) for name in names) File "/trytond/model/fields/function.py", line 133, in call return {r.id: convert(method(r, name), name) for r in records} File "/trytond/model/fields/function.py", line 133, in <dictcomp> return {r.id: convert(method(r, name), name) for r in records} File "/trytond/modules/production_outsourcing/production.py", line 124, in get_cost cost += Currency.compute( TypeError: unsupported operand type(s) for +=: 'decimal.Decimal' and 'NoneType' Fault: unsupported operand type(s) for +=: 'decimal.Decimal' and 'NoneType' ```
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