[FIX] product: default pricelist item

By default, a pricelist can be created without pricelist item. However,
if a user does not have the rights to manage the pricelist items (or
simply does not want to manage them), the feature is unusable. Indeed,
the user will create a new pricelist (for example, in a new currency),
but this pricelist will not have any item. Therefore, the pricelist will
not apply to anything.

The fix creates a pricelist item by default. The pricelist item is the
most generic possible: it applies to all products and uses the list
price as a base price.

opw-667607