[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#84522

X-original-commit: 50b538e7
Signed-off-by: Arnold Moyaux <arm@odoo.com>