[FIX] stock: allow multi-picking validation w/show reception report

Steps to reproduce:
- activate `group_stock_reception_report` in settings
- activate `auto_show_reception_report` for the "Receipts" operation type
- create a receipt w/ any product + confirm
- create a return w/any product + confirm
- select both the receipt + return in the (Operations > Transfers) list
  view > action > Validate

Expected result: both pickings are validated
Actual result: stacktrace due to expected singleton ValueError

enterprise PR : https://github.com/odoo/enterprise/pull/37518


task-3204596

closes odoo/odoo#120159

X-original-commit: b2ac4b2f30e15c4b8a50628798b2fc39d7d43331
Related: odoo/enterprise#40552
Signed-off-by: Tiffany Chang <tic@odoo.com>