FIX supplier price duplicate entry on update supplier product ref

[lvessiller-opendsi]

FIX supplier price duplicate entry on update supplier product ref

• when you edit an existing supplier price and when you update en empty supplier product reference with a new one, you got a duplicate entry SQL error

To reproduce
You have this product supplier price :

You edit this supplier price and set a new product supplier reference : "PROD1FOURN1"

Then when you save this product supplier price, you got this error :

Remarks
This fix is only for products with no supplier reference : the supplier reference of product is empty in database.
The field of "ref_fourn" is not mandatory in database and can be NULL.
So you can have a database with this case.

And for an update you don't need to use 'add_fournisseur()" method because you know the rowid of "product_supplier_price" table. And no need to check if the unique key for supplier reference already exist : it will be return by the SQL server such as "mysql".