Stop error notices in shipping calculator

When load the cart page the following errors appear:

```
<b>Notice</b>:  Array to string conversion in <b>/var/www/woocommerce-dev/wp-includes/formatting.php</b> on line <b>584</b><br />
<option value="AF" >Array</option><br />
<b>Notice</b>:  Array to string conversion in <b>/var/www/woocommerce-dev/wp-includes/formatting.php</b> on line <b>584</b><br />
<option value="AT" >Array</option><br />
<b>Notice</b>:  Array to string conversion in <b>/var/www/woocommerce-dev/wp-includes/formatting.php</b> on line <b>584</b><br />
<option value="BE" >Array</option><br />
<b>Notice</b>:  Array to string conversion in <b>/var/www/woocommerce-dev/wp-includes/formatting.php</b> on line <b>584</b><br />
<option value="BI" >Array</option><br />
....
```

This because the `WC()->customer->get_shipping_country()` is empty: https://github.com/woothemes/woocommerce/blob/master/templates/cart/shipping-calculator.php#L38-L41

To maintain the same behavior we had before we need to test whether this coming empty.
Besides always returns an array to maintain the consistency of the method.