Ensure order meta attached before payment_complete hook

[reykjalin]

Fixes N/A

Changes proposed in this Pull Request

Make sure order meta has been attached before running the woocommerce_payment_complete hook. If the order meta hasn't been attached by the time the hook is run WooPay checkout will not work.

Testing instructions

• Go through a WooPay checkout and make sure it works.

---

• Run npm run changelog to add a changelog file, choose patch to leave it empty if the change is not significant. You can add multiple changelog files in one PR by running this command a few times.
• Covered with tests (or have a good reason not to test in description ☝️)
• Tested on mobile (or does not apply)

Post merge

• Link to testing instructions from release testing doc following these instructions : Add link here / 'QA Testing Not Applicable'
• Add or update critical flows and testing instructions for critical flows, if applicable.

[reykjalin]

Closing in favor of #4284