[FIX] mrp: prevent traceback when choosing a BOM of a product without a variant

Steps to reproduce the bug:
- Go to inventory > configuration > Products > Attributes
- Create a new Dynamic Attributes > add two attribute values
- Create a storable Product “Test”:
    - Add the two attributes
    - Save
    - Create a BOM related to this product
- Create a new manufacturing order:
    - Do not choose a product
    - Select the BOM related to the product “test”

Problem:
Traceback is triggered because as the product template only has dynamic variants, there is not a `product.product` record created yet.
but we try to access it in the onchange: https://github.com/odoo/odoo/blob/14.0/addons/mrp/models/mrp_production.py#L598

Solution:
do not set the product when a BOM of a product_tmpl without a variant was chosen
opw-2732254

closes odoo/odoo#84543

X-original-commit: 50b538e7d32bbce7cad7c05f0bc80db6b969a70b
Signed-off-by: Arnold Moyaux <arm@odoo.com>
Signed-off-by: Djamel Touati (otd) <otd@odoo.com>

addons/mrp/models/mrp_production.py

@@ -639,7 +639,7 @@ class MrpProduction(models.Model):
     @api.onchange('bom_id')
     def _onchange_bom_id(self):
         if not self.product_id and self.bom_id:
-            self.product_id = self.bom_id.product_id or self.bom_id.product_tmpl_id.product_variant_ids[0]
+            self.product_id = self.bom_id.product_id or self.bom_id.product_tmpl_id.product_variant_ids[:1]
         self.product_qty = self.bom_id.product_qty or 1.0
         self.product_uom_id = self.bom_id and self.bom_id.product_uom_id.id or self.product_id.uom_id.id
         self.move_raw_ids = [(2, move.id) for move in self.move_raw_ids.filtered(lambda m: m.bom_line_id)]