Allow setting empty value in woocommerce_checkout_get_value filter hook

There is a flaw in the current implementation of `woocommerce_checkout_get_value` filter hook - it does not allow setting an empty value. If an empty value is set, the execution continues to look for the field value from user profile.

I have reworked the filter hook to be applied after the default value has been selected so that the user can override whatever value was loaded from user profile and provide their own value.

My initial idea was to apply this filter to non-logged-in users as well, but that seemed unnecessary, because there are already filters for loading the default address values for them.