Fixed the issue when saving settings

Hi Tareq!

My client use this plugin on one of the site along with the WooCommerce Ambassador plugin which also use the WC_Integration class.

The problem is that when the settings of the Ambassador plugin are saved, the action `woocommerce_update_options_integration` is triggered as well, there are of course fields of your plugin missing and I get the following notice and all the fields of your plugin are reset to blank: `Notice: Undefined index: woocommerce_wc_conv_tracking_cart in [...]/wp-content/plugins/woocommerce-conversion-tracking/includes/integration.php on line 79`.

The solution was simple and I already implemented it. Just one IF statement was missing to check the hidden field of section. So the action is only triggered we are in the right section as well to update the settings.

I'd love to see this issue merged into master :)