[FIX] purchase: Wrong quantity set up with packaging

Steps to reproduce the issue:

    - Create a product P
    - Enable packaging on Purchase
    - Create a packaging PA such as 1 PA of P = 70 P
    - Create a PO with P and set a quantity of 250 with PA as packaging

Bug:

    A Warning was raised saying that we should purchase 280, PA was removed and a
    quantity of 249.9 was set.

PS: PA was removed because the quantity was changed and function _onchange_suggest_packaging was triggered

opw:3007165

closes odoo/odoo#105065

X-original-commit: a9514d69006c8a389b64550dbcbf30b19160c300
Signed-off-by: William Henrotin (whe) <whe@odoo.com>

addons/purchase/models/purchase.py

@@ -1250,7 +1250,7 @@ class PurchaseOrderLine(models.Model):
                 line.product_packaging_id = False
             # suggest biggest suitable packaging
             if line.product_id and line.product_qty and line.product_uom:
-                line.product_packaging_id = line.product_id.packaging_ids.filtered('purchase')._find_suitable_product_packaging(line.product_qty, line.product_uom)
+                line.product_packaging_id = line.product_id.packaging_ids.filtered('purchase')._find_suitable_product_packaging(line.product_qty, line.product_uom) or line.product_packaging_id
 
     @api.onchange('product_packaging_id')
     def _onchange_product_packaging_id(self):