Remove unique constraint from order_key to prevent empty key conflict.

[vedanshujain]

Submission Review Guidelines:

• I have followed the WooCommerce Contributing Guidelines and the WordPress Coding Standards.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• I have reviewed my code for security best practices.
• Following the above guidelines will result in quick merges and clear and detailed feedback when appropriate.

Changes proposed in this Pull Request:

While order_key is generally expected to be unique per order, it looks like it's not necessarily true, and orders can have no keys at all. In this case, wpdb->prepare converts a null value to an empty string, which conflicts with existing orders that also don't have any order key.

This PR removes the unique constraint and makes it consistent with the post store. Note that migration for existing orders will be tackled in a different PR.

How to test the changes in this Pull Request:

Using the WooCommerce Testing Instructions Guide, include your detailed testing instructions:

Delete the orders table by going to WooCommerce > Status > Tools. Create the tables again, verify that unique constraint is removed.

[github-actions]

Test Results Summary

Commit SHA: 408cf92

Test 🧪	Passed ✅	Failed 🚨	Broken 🚧	Skipped ⏭️	Unknown ❔	Total 📊	Duration ⏱️
API Tests	259	0	0	2	0	261	0m 51s
E2E Tests	186	0	0	10	0	196	13m 49s

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To view the full E2E test report, click here.
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[github-actions]

Hi @barryhughes,

Apart from reviewing the code changes, please make sure to review the testing instructions as well.

You can follow this guide to find out what good testing instructions should look like:
https://github.com/woocommerce/woocommerce/wiki/Writing-high-quality-testing-instructions

[barryhughes]

Looks good, one minor tweak to consider re changelog messaging.

I also wondered, should we have a migration in relation to this, or is that non-essential at this stage?

[vedanshujain]

I also wondered, should we have a migration in relation to this, or is that non-essential at this stage?

I am working on an optional migration for both this and #37676 in a separate PR.