Fixes saving product in single-store mode if website_id <> 1

[eduard13]

Description

This PR fixes the saving product in a single store mode, where the default website was removed, and a new one it is used instead.

Fixed Issues (if relevant)

1. No such entity error when saving product in single-store mode if website_id <> 1 #13405: No such entity error when saving product in single-store mode if website_id <> 1

Manual testing scenarios

1. Install a clean version of Magento version 2.2.2
2. Add a new website, store and store view, and make this the default
3. Delete the preinstalled website, store and store view
4. Add a new product and save it
5. Change the configurations to single-store mode
6. Open the product and save it again

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 @eduard13. 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 {$VERSION} instance - deploy vanilla Magento instance

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

[rogyar]

I just wanted to raise my concerns regarding the singleStoreMode condition absence, but you have already fixed this point :) Let's see the test results

[rogyar]

@magento-engcom-team give me 2.2.5 instance

[magento-engcom-team]

Hi @rogyar. Thank you for your request. I'm working on Magento 2.2.5 instance for you

[magento-engcom-team]

Hi @rogyar, here is your Magento instance.
Admin access: https://i-18001-2-2-5.engcom.dev.magento.com/admin
Login: admin Password: 123123q
Instance will be terminated in up to 3 hours.

[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-18001.engcom.dev.magento.com/admin
Login: admin Password: 123123q

[magento-engcom-team]

Hi @eduard13. Thank you for your contribution.
We will aim to release these changes as part of 2.2.8.
Please check the release notes for final confirmation.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.

[magento-engcom-team]

Hi @eduard13. Thank you for your contribution.
We will aim to release these changes as part of 2.2.8.
Please check the release notes for final confirmation.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.