[Issue] Avoid exception when config.xml nodes exist for not-installed payment methods

[m2-assistant]

This issue is automatically created based on existing pull request: #27940: Avoid exception when config.xml nodes exist for not-installed payment methods

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Preconditions (*)

Magento 2.4-develop
When a payment configuration node exists in XML but there is no <model> defined, getList() will throw an UnexpectedValueException. One use case for creating such a set-up is a module that provides default configuration overrides for payment methods which are not installed on the current website. For example:

<config>
  <default>
    <payment>
      <checkmo>
        <active>0</active>
      </checkmo>
      <not_installed_here>
        <debug>0</debug>
        <environment>production</environment>
      </not_installed_here>
    </payment>
  </default>
</config>

In this case, the payment method not_installed_here does not have a <model> node defined, as the payment method module is not installed on the website.

Steps to reproduce (*)

1. Add supplied XML or similar to a local module's config.xml
2. Browse to checkout with an item in basket
3. Observe (lack of) exception thrown during checkout page load

Expected result (*)

Actual result (*)

Additional information
I have not yet written/changed any unit/functional tests for this change. I suspect some existing tests may fail which I will review at a later date.