[FIX] onchange_helper: Do not overwrite precomputed fields

[SirAionTech]

Steps:

1. Install sale_management
2. Create a product template product
3. Create a pricelist with discount_policy = without_discount, with a rule for the created product
4. Create a sale order order
5. In the Python code, play the onchanges for product_id for a new line having values:

   {
       "order_id": order.id,
       "product_id": product.id,
   }

Observed behavior:
Error

Details

File "/home/odoo/src/user/OCA/server-tools/onchange_helper/models/base.py", line 58, in play_onchanges
    onchange_values = self.onchange(all_values, field, onchange_specs)
  File "/home/odoo/src/odoo/odoo/models.py", line 6622, in onchange
    todo = [
  File "/home/odoo/src/odoo/odoo/models.py", line 6625, in 
    if name not in done and snapshot0.has_changed(name)
  File "/home/odoo/src/odoo/odoo/models.py", line 6408, in has_changed
    return self[name] != record[name]
  File "/home/odoo/src/odoo/odoo/models.py", line 5957, in __getitem__
    return self._fields[key].__get__(self, self.env.registry[self._name])
  File "/home/odoo/src/odoo/odoo/fields.py", line 1158, in __get__
    self.recompute(record)
  File "/home/odoo/src/odoo/odoo/fields.py", line 1370, in recompute
    apply_except_missing(self.compute_value, recs)
  File "/home/odoo/src/odoo/odoo/fields.py", line 1343, in apply_except_missing
    func(records)
  File "/home/odoo/src/odoo/odoo/fields.py", line 1392, in compute_value
    records._compute_field_value(self)
  File "/home/odoo/src/odoo/odoo/models.py", line 4241, in _compute_field_value
    fields.determine(field.compute, self)
  File "/home/odoo/src/odoo/odoo/fields.py", line 98, in determine
    return needle(*args)
  File "/home/odoo/src/user/OCA/sale-workflow/sale_pricelist_triple_discount/model/sale.py", line 29, in compute_discount
    res = super()._compute_discount()
  File "/home/odoo/src/odoo/addons/sale/models/sale_order_line.py", line 583, in _compute_discount
    base_price = line._get_pricelist_price_before_discount()
  File "/home/odoo/src/odoo/addons/sale/models/sale_order_line.py", line 551, in _get_pricelist_price_before_discount
    price = pricelist_rule._compute_base_price(
  File "/home/odoo/src/odoo/addons/product/models/product_pricelist_item.py", line 422, in _compute_base_price
    target_currency.ensure_one()
  File "/home/odoo/src/odoo/odoo/models.py", line 5186, in ensure_one
    raise ValueError("Expected singleton: %s" % self)
ValueError: Expected singleton: res.currency()

Expected behavior:
The order line is created

Additional context:
I could not add a test case for this module due to the complexity of sale model and precomputed fields, even using fake models of https://github.com/OCA/odoo-test-helper, any suggestion is appreciated.
The only test case I managed to create is in OCA/product-configurator#145.