Fix - Out of stock stock products in WooCommerce remain listed on Facebook

[rawdreeg]

Changes proposed in this Pull Request:

If a user has checked the "Hide out of stock items from the catalog" option in WooCommerce (WooCommerce -> settings -> Products -> inventory), out-of-stock variable products are not deleted from Facebook. This PR fixes the logic to ensure the FB product is removed.

Closes #2277.

• [ x] Do the changed files pass phpcs checks? Please remove phpcs:ignore comments in changed files and fix any issues, or delete if not practical.

Detailed test instructions:

1. Make sure the "Hide out of stock items from the catalog" option in WooCommerce (WooCommerce -> settings -> Products -> inventory) is checked
2. Edit a simple product and set the stock quantity to 0.
3. The product should be removed from the FB catalog.
4. Repeat steps 2 and 3 with a variable product and observe that the product is removed from the catalog

Changelog entry

Fix - Remove out-of-stock products on Facebook when the "Hide out of stock items from the catalog" option in WooCommerce is checked.

[message-dimke]

LGTM! Thanks, @rawdreeg