Don't make ajax request for provinces if contry value is empty

When entering in a page whit address involved like checkout addressing step or create new address, you get 404 error in chrome dev when province-choice.js does the ajax request to get the provinces of the selected country.

This patch changes the null comparation to ```''``` and also fades out the provinceContainer in case user selects first option of the combo countries again. 
Edit: Maybe created a dedicate function for this in order to not repeat code?