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'
```
issue