Fix error visiting compare page without items

[indykoning]

Description (*)

Going incognito and visiting /catalog/product_compare/index/ will cause an

Item (Magento\Catalog\Model\Product\Interceptor) with the same ID "xxx" already exists. 

Error on many live webshops. This is because the database table looks somewhat like this

catalog_compare_item_id	visitor_id	customer_id	product_id	store_id	list_id
23159	4690502		19028	1
22917	0		18906	1	711
23034	0		18868	1	732
22916	0		18851	1	711
21991	0		18783	1	537
21573	0		18747	1	451
22196	0		18740	1	585
22485	0		18706	1	644
22111	0		18705	1	568
22259	0		18689	1	596
22133	0		18689	1	573
21802	0		18689	1	492

Thus if we only get items with visitor_id 0 and customer_id NULL we will get duplicate products (and compare product from other customers...)

Manual testing scenarios (*)

1. Ensure your catalog_compare_items somewhat matches the example above (duplicate product id's on different lists)
2. Visit the /catalog/product_compare/index/ route in incognito, so no visitor id, customer id, or list id is assigned to you
3. See the error that occurs.

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)
• README.md files for modified modules are updated and included in the pull request if any README.md predefined sections require an update
• All automated tests passed successfully (all builds are green)

Resolved issues:

1. resolves [Issue] Fix error visiting compare page without items #40205: Fix error visiting compare page without items

[m2-assistant]

Hi @indykoning. Thank you for your contribution!
Here are some useful tips on how you can test your changes using Magento test environment.
❗ Automated tests can be triggered manually with an appropriate comment:

• @magento run all tests - run or re-run all required tests against the PR changes
• @magento run <test-build(s)> - run or re-run specific test build(s)
  For example: @magento run Unit Tests

<test-build(s)> is a comma-separated list of build names.

Allowed build names are:

1. Database Compare
2. Functional Tests CE
3. Functional Tests EE
4. Functional Tests B2B
5. Integration Tests
6. Magento Health Index
7. Sample Data Tests CE
8. Sample Data Tests EE
9. Sample Data Tests B2B
10. Static Tests
11. Unit Tests
12. WebAPI Tests
13. Semantic Version Checker

You can find more information about the builds here
ℹ️ Run only required test builds during development. Run all test builds before sending your pull request for review.

---

For more details, review the Code Contributions documentation.
Join Magento Community Engineering Slack and ask your questions in #github channel.

[engcom-Hotel]

@magento create issue