**Issue**
When a subscription order line is deleted after being invoiced, attempting to create a new invoice for the subscription raises a UserError about UoM category mismatch.
Video: https://drive.google.com/file/d/11-CV7wcEHQJFoVEQM5o5YBkZuTXPYDqL/view

**Steps to Reproduce**
1. Create and confirm a subscription with a recurring product (e.g., Car Leasing)
2. Generate and post the invoice for the subscription
3. Add a new product line to the subscription (e.g., Office Cleaning Service)
4. Delete the original invoiced line (Car Leasing)
5. Attempt to create an invoice for the new product line
→ Error: "The unit of measure Units defined on the order line doesn't belong to the same category as the unit of measure False defined on the product. Please correct the unit of measure defined on the order line or on the product, they should belong to the same category."
(https://drive.google.com/file/d/11-CV7wcEHQJFoVEQM5o5YBkZuTXPYDqL/view)

**Root Cause**
Commit https://github.com/odoo/enterprise/commit/22e49fca1e0fbfefac974c562491d170b8d70025 introduced quantity tracking per period in _get_max_invoiced_date() to fix partial credit note handling. The implementation accesses `sale_line_ids.product_uom` assuming sale_line_ids is always populated. However, when a sale order line is deleted, the related account.move.line remains in the system with empty sale_line_ids. Accessing `sale_line_ids.product_uom` on an empty recordset returns False, causing the UoM validation to fail during invoice creation.

**Fix**
Add a fallback to use the invoice line's own product_uom_id when sale_line_ids is empty. This preserves the partial credit note fix from https://github.com/odoo/enterprise/commit/22e49fca1e0fbfefac974c562491d170b8d70025 while handling the edge case of deleted subscription lines. If no valid UoM is found, the line is skipped in the calculation.