Fix is_empty null Error

[n7studios]

Summary

Fixes this reported issue, where the WooCommerce cart isn't available:

An error of type E_ERROR was caused in line 80 of the file /home/tqhmcezb/domains/yujenu.wordifysites.com/public_html/wp-content/plugins/convertkit-for-woocommerce/includes/class-ckwc-abandoned-cart.php. Error message: Uncaught Error: Call to a member function is_empty() on null in /home/tqhmcezb/domains/yujenu.wordifysites.com/public_html/wp-content/plugins/convertkit-for-woocommerce/includes/class-ckwc-abandoned-cart.php:80

Testing

Existing tests pass.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

[ciccio-kit]

LGTM ✅