[FIX] account_reconcile_model_oca: Fix unassigned variable 'balance'

It seems some people are getting an "unassigned variable 'balance'".
This shouldn't happen with the standard code because _get_write_off_move_lines_dict is only
called from the manual reconciliation widget in which, you only have access to models having
strictly lines being percentage or fixed (see get_reconcile_modelds_for_manual_reconciliation).
This method is not called from the bank reco widget pointed by "RECONCILE x ITEMS" on the accounting dashboard.
It probably comes from some customization but I don't know which ones.

Related to 99525f8dac

account_reconcile_model_oca/models/account_reconcile_model.py

@@ -131,6 +131,8 @@ class AccountReconcileModel(models.Model):
                 balance = currency.round(
                     line.amount * (1 if residual_balance > 0.0 else -1)
                 )
+            else:
+                balance = 0.0
 
             if currency.is_zero(balance):
                 continue