[IMP] point_of_sale: check constraint at session creation

The constraints could be bypassed if a user set a jourmal/account/location to
the pos.config and, then, modifies the location configuration.

Checking the value of current_session_id makes sure the constraints is triggered
also when the session is created.

Prevents workarounds such as explained at #26641

addons/point_of_sale/models/pos_config.py

@@ -437,6 +437,11 @@ class PosConfig(models.Model):
         """
         self.ensure_one()
         if not self.current_session_id:
+            self._check_company_location()
+            self._check_company_journal()
+            self._check_company_invoice_journal()
+            self._check_company_payment()
+            self._check_currencies()
             self.current_session_id = self.env['pos.session'].create({
                 'user_id': self.env.uid,
                 'config_id': self.id