Warning: Undefined Index ‘refersion_status’

Hello,

I’d like to report a small bug that’s causing PHP warnings when Refersion is set to “No, turn off Refersion reporting”.

On the front-end of my site, I’m getting the warning: “Undefined index: refersion_status” on lines 125 and 150 of /refersion-for-woocommerce/class.refersion.php

https://plugins.trac.ww.wp.xz.cn/browser/refersion-for-woocommerce/trunk/class.refersion.php#L125
https://plugins.trac.ww.wp.xz.cn/browser/refersion-for-woocommerce/trunk/class.refersion.php#L150

This problem only happens when I select “No, turn off Refersion reporting” in the admin panel.

I suspect that it is happening because of the use of !empty() in the sanitize function on line 241 of class.refersion-admin.php

https://plugins.trac.ww.wp.xz.cn/browser/refersion-for-woocommerce/trunk/class.refersion-admin.php#L241

The select option for “No, turn off Refersion reporting” is zero and the integer zero counts as empty.

I locally changed this !empty() to isset() and the zero was correctly saved to the database, so $options['refersion_status'] becomes a defined index.

Thanks!