[FIX] fleet: avoid singleton error

When doing a write on multiple records, message_post expects a single record
if self.ids:
self.ensure_one()

closes odoo/odoo#33645

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>

addons/fleet/models/fleet_vehicle.py

@@ -225,7 +225,7 @@ class FleetVehicle(models.Model):
                 changes.append(_("License Plate: from '%s' to '%s'") % (old_license_plate, vals['license_plate']))
 
             if len(changes) > 0:
-                self.message_post(body=", ".join(changes))
+                vehicle.message_post(body=", ".join(changes))
 
             return super(FleetVehicle, self).write(vals)