[Forwardport] Fix Customer custom attributes lost after save

[gelanivishal]

Original Pull Request

#17968

Description

The customer model has an attribute attribute_set_id that links to its attribute set. It is used while saving, as there is a check to see if its attributes are in the set or not, using this link.
We want to avoid a null attribute_set_id.

Fixed Issues

1. Saving Customer Model directly causes loss of data #12479: Saving Customer Model directly causes loss of data

Manual testing scenarios

1. Disable EAV cache.
2. Create a custom attribute for Customer entity.
3. Update a customer in the admin and add a value for this new attribute.
4. Programmatically load the customer, make some change (e.g. firstname) and then save directly using the model.
5. Reload the customer: the custom value is not lost.

No regression manual test:
6. Programmatically load the customer, add a different not-yet-defined attribute to customer (e.g. setNotYetDefined(1)) and save directly using the model.
7. Reload the customer: the custom value is still not lost, while not_yet_defined is lost.

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)

[magento-engcom-team]

Hi @gelanivishal. Thank you for your contribution
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Add the comment under your pull request to deploy test or vanilla Magento instance:

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[magento-engcom-team]

Hi @sidolov, thank you for the review.
ENGCOM-3181 has been created to process this Pull Request

[magento-engcom-team]

Hi @gelanivishal. Thank you for your contribution.
We will aim to release these changes as part of 2.3.1.
Please check the release notes for final confirmation.

[magento-engcom-team]

Hi @gelanivishal. Thank you for your contribution.
We will aim to release these changes as part of 2.3.1.
Please check the release notes for final confirmation.