[17.0][FIX] product_secondary_unit: ensure qty persists.

[Qlasta]

Problem occurred after sale_stock module update.
task id - 26079

Description
After new write method update in sale.order.line, line change triggers sec uom qty calculation and test fails in sale_order_secondary_unit module.

Current Behaviour
Test fails:

2025-06-11 03:44:49,795 1 INFO odoodb odoo.addons.sale_order_secondary_unit.tests.test_sale_order: ======================================================================
Error: 2025-06-11 03:44:49,795 1 ERROR odoodb odoo.addons.sale_order_secondary_unit.tests.test_sale_order: FAIL: TestSaleOrder.test_independent_type

"/opt/odoo/projects/sale-workflow/sale_order_secondary_unit/tests/test_sale_order.py", line 95, in test_independent_type
   self.assertEqual(self.order.order_line.product_uom_qty, previous_uom_qty)
pr=2890#step:8:3432)AssertionError: 1.0 != 0.0

Expected Behavior
Test does not fail. Setting secondary_uom_id on line (at the moment when secondary qty is not set), won't reset existing product_uom_qty .

Proposed implementation
Check if secondary_uom_qty exists, if not - do not continue calculation and do not reset qty.

[oerp-odoo]

@Qlasta we should now create pull requests targeting OCA repo. And then use git-aggregator to combine it into final aggregated branch (you can see examples in repos.yml). So we should avoid 17.0-versada branches.

[Qlasta]

PR created on OCA repo