[FIX]sale_stock: Issue when clicking on "No backorder"

Steps to reproduce the bug:
- Create a SO with no salesperson
- Add a SO line with 2 storable product
- Confirm the SO and deliver only one product
- Click on "No backorder"

Bug:

A error was raised because the function "_log_activity"(called by _log_less_quantities_than_expected) requires a responsible

Partial Backport: https://github.com/odoo/odoo/commit/e1f9499d2672fb0a346f8ac74552f038f5204a64



opw:1950968

closes odoo/odoo#32592

Signed-off-by: Simon Goffin (sig) <sig@openerp.com>