Fixing add to cart when no custom option is passed through the form

[jeromin]

When no custom option data is sent through the add to cart form, it raised an error as the foreach variable is null.

Warning: foreach() argument must be of type array|object, null given

Just adding the same check as the listener above.

[mamazu]

Looks good. Thanks for contributing.

[jeromin]

Thank you for merging my very first PR in the Open Source world 😁