Fix: Deleting a shop coupon using API returns a 500 error

[vedanshujain]

Copied over from #47199 to add a unit test and a changelog.

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:

Closes #45829 .

How to test the changes in this Pull Request:

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

1. Create a coupon and note it's ID (or use an existing coupon).
2. Delete the coupon via REST API, like so: /wp-json/wc/v3/coupons/COUPON_ID. Make sure that there is no 500 error.

Changelog entry

• Automatically create a changelog entry from the details below.

Significance

• Patch
• Minor
• Major

Type

• Fix - Fixes an existing bug
• Add - Adds functionality
• Update - Update existing functionality
• Dev - Development related task
• Tweak - A minor adjustment to the codebase
• Performance - Address performance issues
• Enhancement - Improvement to existing functionality

Message

Fix coupon REST API error when deleting a coupon.

Comment

[github-actions]

Test using WordPress Playground

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Test this pull request with WordPress Playground.

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[github-actions]

Hi @naman03malhotra,

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

[naman03malhotra]

Tests well, works as expected! 🚢