Fix - Fatal error when facebook_config option is empty.

[rawdreeg]

Changes proposed in this Pull Request:

We received a report of a fatal error in the Pixel Event class. After investigation, I noticed that if the facebook_config option is an empty string, the value returned by get_options would be an empty string, causing the exception: error message: Uncaught TypeError: Cannot access offset of type string on string.

This assumes that facebook_config will always contain a serialized object, and the error is, in fact, caused by the facebook_config option being empty.

Closes #2516.

• 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. log in as an admin, and navigate to wp-admin/options.php then delete the content of facebook_config.
2. navigate to any admin page, and you should receive the fatal error.
3. Switch to this branch, and the fatal should be gone.

Changelog entry

Fix - Fatal error when facebook_config option is empty.

[budzanowski]

Nice!

[ecgan]

@rawdreeg , in the issue #2516, the users mentioned the issue only happens when they upgraded to use PHP 8.1, it did not happen when they were using PHP 7.4. Do you know why? 🤔