Fixed assignment of the guest customer to the guest group when 'Automatic Assignment by VAT ID' is enabled

[vovayatsyuk]

Description

Current logic doesn't work for a guest group id since it's 0.

This commit fixes the case when guest fills valid VAT number,
then returns to the shipping step and removes VAT from the address.

Manual testing scenarios

1. Navigate to Customers > Customer Groups and create two groups "Valid VAT", "Invalid VAT".
2. Navigate to Stores > Configuration > Customers > Customer Configuration > Create New Account Options and enable "Enable Automatic Assignment to Customer Group". Use groups from the first step for valid and invalid vat id groups. Don't forget to enable "Show VAT Number on Storefront" option.
3. Open checkout as a guest and use "LU26375245" as a VAT number and "Luxembourg" as a country.
4. Proceed to the payment step.
5. Open quote DB table using adminer of phpmyadmin, find your quote and look at the customer_group_id column. You'll see id of valid group (4, for example).
6. Return to the shipping step, remove VAT number, and proceed to the payment step again.
7. Take a look and the quote table again. I'm expecting to see 0 here but it still has 4.

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 on Travis CI are green)

[m2-assistant]

Hi @vovayatsyuk. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento-engcom-team give me test instance - deploy test instance based on PR changes
• @magento-engcom-team give me 2.3-develop instance - deploy vanilla Magento instance

For more details, please, review the Magento Contributor Assistant documentation

[rogyar]

@magento-engcom-team give me test instance

[magento-engcom-team]

Hi @rogyar. Thank you for your request. I'm working on Magento instance for you

[magento-engcom-team]

Hi @rogyar, here is your new Magento instance.
Admin access: https://pr-22091.instances.magento-community.engineering/admin
Login: admin Password: 123123q

[magento-engcom-team]

Hi @rogyar, thank you for the review.
ENGCOM-4635 has been created to process this Pull Request

[VasylShvorak]

✔️ QA passed

[m2-assistant]

Hi @vovayatsyuk, thank you for your contribution!
Please, complete Contribution Survey, it will take less than a minute.
Your feedback will help us to improve contribution process.