[16.0][FIX] contract: proper condition for auto price

[rvalyi]

IMPORTANT EDIT: the bug is present on 16.0 but it is not present in 15.0 nor 17.0. So no port will be needed for these versions.

without these conditions, if you create a new contract template with a new line with the price set as "automatic" (checkbox) while the contract has no pricelist, then you get this error (easy to reproduce on the Runboat):

  File "/opt/odoo/odoo/models.py", line 6622, in onchange
    todo = [
  File "/opt/odoo/odoo/models.py", line 6625, in <listcomp>
    if name not in done and snapshot0.has_changed(name)
  File "/opt/odoo/odoo/models.py", line 6408, in has_changed
    return self[name] != record[name]
  File "/opt/odoo/odoo/models.py", line 5957, in __getitem__
    return self._fields[key].__get__(self, self.env.registry[self._name])
  File "/opt/odoo/odoo/fields.py", line 1210, in __get__
    self.compute_value(recs)
  File "/opt/odoo/odoo/fields.py", line 1392, in compute_value
    records._compute_field_value(self)
  File "/opt/odoo/odoo/models.py", line 4241, in _compute_field_value
    fields.determine(field.compute, self)
  File "/opt/odoo/odoo/fields.py", line 98, in determine
    return needle(*args)
  File "/mnt/data/odoo-addons-dir/contract/models/abstract_contract_line.py", line 214, in _compute_price_unit
    line.price_unit = pricelist._get_product_price(product, quantity=1)
  File "/opt/odoo/addons/product/models/product_pricelist.py", line 84, in _get_product_price
    self.ensure_one()
  File "/opt/odoo/odoo/models.py", line 5186, in ensure_one
    raise ValueError("Expected singleton: %s" % self)
ValueError: Expected singleton: product.pricelist()

[rvalyi]

IMPORTANT NOTE: the bug is present on 16.0 but it is not present in 15.0 nor 17.0. So no port will be needed for these versions.

[OCA-git-bot]

This PR has the approved label and has been created more than 5 days ago. It should therefore be ready to merge by a maintainer (or a PSC member if the concerned addon has no declared maintainer). 🤖

[pedrobaeza]

/ocabot merge minor

Please fw-port it to 17

[OCA-git-bot]

This PR looks fantastic, let's merge it!
Prepared branch 16.0-ocabot-merge-pr-1158-by-pedrobaeza-bump-minor, awaiting test results.

[OCA-git-bot]

Congratulations, your PR was merged at 5d5ab04. Thanks a lot for contributing to OCA. ❤️

[rvalyi]

/ocabot merge minor

Please fw-port it to 17
@pedrobaeza as I commented the bug is not present in v15 nor v17... It's probably related to the way fields are computed. Do you want me to put a blacklist in the oca-port metadata? I can also forward-port it, it is harmless but also useless.

[pedrobaeza]

OK, don't worry then.