Fix productlink collection provider missing items

[tdgroot]

Fix productlink collection provider missing items when some positions are null.

Description

There were several issues with the productlink collection provider.

When several items have the position value of null, the position will get converted to '0'. When sorting, the items get sorted by indexing the items by position value and sort them by a key sort. Multiple '0' values for position, means duplicate indexes.

The same applies with every case where multiple items have the same position value.

Manual testing scenarios

1. Create some configurable products
2. Add related products to one of them
3. Try sorting the related products

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]

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[sidolov]

Hi @tdgroot , looks like changes related to your PR was merged to 2.3-develop. Please, resolve merge conflicts and check if the issue still present.
Thank you!

[tdgroot]

@sidolov thank you for informing. I think @hostep's work solves my problem! Will check soon.

[hostep]

Nice @tdgroot 👍 , our changes are nearly identical, makes me feel more confident that the solution is a good one 🙂
Feel free to double check though.

[tdgroot]

@hostep exactly what I was thinking! Magento contribution are great sanity checks for your solutions :).

[tdgroot]

Fixed in #18207