Fix a bug that calling split() on null value in the useStoreAddress hook

[eason9487]

Changes proposed in this Pull Request:

• Fix a bug that calling split() on null value in the useStoreAddress hook.
• Make sure the returned data types of useStoreAddress hook are consistent.

Detailed test instructions:

1. Clear store address via the WC Settings page or SQL:
   

   DELETE FROM `wp_options` WHERE `option_name` IN ('woocommerce_store_address', 'woocommerce_store_address_2', 'woocommerce_store_city', 'woocommerce_default_country', 'woocommerce_store_postcode');

2. Go to GLA's MC setup or settings pages.
3. It should be able to render pages without the Cannot read property 'split' of null error.
4. The returned fields in the data property of useStoreAddress hook should always be string types.

Changelog entry

[ecgan]

Tested, working as expected, LGTM 👍

[ecgan]

I thought about whether want to specify fallback '' here in case wcAddress.country does not exist in countryNameDict, but in the end that is not necessary because users can only select a value from the predefined values in the country dropdown and cannot enter a value of their own, so there shouldn't be any problem.