Handle missing states and set a default value

MWS Addresses require a StateOrProvinceCode though not
all countries (such as France) have states. If the code is
missing, assume that it is because the country doesn't have
this and set the code to N/A

This a fix for https://github.com/Shopify/shopify/issues/18877

Please review @cjoudrey @jeromecornet 

Once I get this into master I'll update the Shopify gemfile accordingly and we should be good to go.