Allow <select> ie no month in the Holiday Carry Over Expiry Month list

We set up a location to have a month expiry, now that we don't want that expiry at all we can't just unselect, we have to create a whole new "location" just to be change one item. It's a huge problem to change the whole profile name and try and delete the old name - when actually we want to use the old name ie it's the name of a country so we can't very well change it. It has implications for other users who use APIs to take our information.