This commit will fixed the error when product has some stock quants
and product category with 'Automated' valuation method, is set to that product.

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Steps to product the above mentioned error:
1. Create product with any category which has 'Manual' valuation method.
2. Update 'On Hand' quantity for the same product.
3. Create product category with valuation method set to 'Automated'.
4. Now set the above product category to the same product which you created in step 1.
5. And Click on the save button.

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Also edit the test case.

closes odoo/odoo#43721

Task: 2155804
Signed-off-by: Simon Lejeune (sle) <sle@openerp.com>