Recompute fixed cost price may return None

When a new company is created, the cost_price of products may be None but recompute_cost_price_fixed return the cost price of the product. This leads to such traceback:

Traceback (most recent call last):
  File "/tryton/trytond/trytond/worker.py", line 135, in run_task
    task.run()
  File "/tryton/trytond/trytond/ir/queue_.py", line 178, in run
    getattr(Model, self.data['method'])(
  File "/tryton/trytond/trytond/modules/stock/product.py", line 321, in recompute_cost_price
    cost = round_price(cost)
           ^^^^^^^^^^^^^^^^^
  File "/tryton/trytond/trytond/modules/product/product.py", line 49, in round_price
    return value.quantize(
           ^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'quantize'

Indeed the other recompute_cost_price_* are all returning 0 by default.