Fix notice that occurs from external function call

This fixed a notices which displays when calling WC_Settings_Integrations->get_sections from a plugin, and there are no integrations.

`Notice: Trying to get property of non-object in /srv/www/wordpress-ms/docroot/wp-content/plugins/woocommerce/includes/admin/settings/class-wc-settings-integrations.php on line 48`