[FIX] repair: display correct default location

How to reproduce the problem:
- Install the repair App
- Repairs -> Create (a Repair Order) (and activate the debug mode)
- Change the Location Field to something else (than the usual default WH/Stock)
- Open Developer Tools -> Set Defaults
- For Defaults, choose "Location = [the changed Location]", "All Users" -> Save Default
- Create a new Repair Order: the Location is not set to the default we set earlier through the Developer Tools

Cause of the problem : an Onchange method was overriding the default Location

Solution : it will now check, in the onchange, if the change is necessary, before overriding the default.

opw-2545876

closes odoo/odoo#74847

Signed-off-by: Rémy Voet <ryv-odoo@users.noreply.github.com>