[Issue] Fixing bug with non-existent customer group breaking quote

[m2-assistant]

This issue is automatically created based on existing pull request: #28902: Fixing bug with non-existent customer group breaking quote

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Preconditions (*)

There's a bug when a customer group deleted, it's still referenced in active quotes which causes an error on the checkout page.
Such a quote becomes broken and the customer is not able to place an order. This pull request adds functionality for getting customer group from the current customer in case customer group saved to the quote does not exist.

Steps to reproduce (*)

1. Create a new customer group.
2. Create a new customer. Link that customer to the customer group created on the previous step.
3. Log in to the Magento storefront and add some products to the shopping cart.
4. Go to the admin and delete the customer group created on step 1.
5. Return to the storefront and proceed to checkout.

Actual result: ✖️ an error "No such entity id = ... " displayed. Shipping methods are not rendered.

Expected result: ✔️ a customer should be able to complete an order without any issue.

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds are green)