Prevented warnings when countries is empty in shipping methods.

Stopped this erros:

PHP Warning:  array_intersect(): Argument #1 is not an array in
wp-content/plugins/woocommerce/includes/abstracts/abstract-wc-shipping-method.php
on line 182
PHP Warning:  in_array() expects parameter 2 to be array, null given in
wp-content/plugins/woocommerce/includes/abstracts/abstract-wc-shipping-method.php
on line 194

@mikejolley