Ensure some wpdb properties exist before accessing them

[lsinger]

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:

Fixes test failures that seem to be PHP 8.0 + WordPress 6.4 related. See e.g. this test run.

How to test the changes in this Pull Request:

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

1. review change; ensure CI passes

Note: the e2e tests appear to be broken right now.

[github-actions]

Test Results Summary

Commit SHA: 1860c4b

Test 🧪	Passed ✅	Failed 🚨	Broken 🚧	Skipped ⏭️	Unknown ❔	Total 📊	Duration ⏱️
API Tests	259	0	0	2	0	261	0m 36s
E2E Tests	209	5	0	1	0	215	23m 30s

⚠️ Warning

Please address the following issues prior to merging this pull request:

• FAILED/BROKEN TESTS. There were failed and/or broken API and E2E tests.
• INCOMPLETE E2E TEST RUN. We have a total of 227 E2E tests, but only 215 were executed. Note that in CI, E2E tests automatically end when they encounter too many failures.

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To view the full API test report, click here.
To view the full E2E test report, click here.
To view all test reports, visit the WooCommerce Test Reports Dashboard.

[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]

LGTM!

Left a possible/optional change, but it's not a blocker. Approving but not merging (since, as you noted, we currently have a problem with one of our required checks).

[beaulebens]

Note that this mysqli property in particular is being reverted in WP: https://make.wordpress.org/core/2023/11/08/wordpress-6-4-1-rc1-is-now-available/

[lsinger]

This PR is now being shipped as part of #41269. This was required to make the unit tests pass in the latter.