[FIX] product_expiry: expiration date on receipt

How to reproduce:
    - Create a tracked product using expiration dates and don't set the
      `expiration_time` field;
    - Create a receipt for this product (using "Create New Lots/Serial
      Numbers") and confirm it;
    - Create a move line for this product and define an expiration date;
    - Valid the picking -> Traceback.

The issue was as `expiration_time` isn't set on product, the production
lot created on the receipt's validation don't have an `expiration_date`.
So, if the `expiration_date` is set on the receipt move line, the
production lot will try to get a time delta with its unset
`expiration_date`, causing the traceback.

task-2119330

closes odoo/odoo#39795

Signed-off-by: Arnold Moyaux <amoyaux@users.noreply.github.com>