Add empty? check method

Solidus has deprecated the `empty?` method on
solidusio/solidus#1686 . This adds a private
`empty?` method for avoid future problems and remove the deprecation
warning in projects that uses this gem.

lib/super_good/solidus_taxjar/tax_calculator.rb

@@ -12,7 +12,7 @@ def initialize(order, api: self.class.default_api)
 
       def calculate
         return no_tax if SuperGood::SolidusTaxJar.test_mode
-        return no_tax if order.tax_address.empty? || order.line_items.none?
+        return no_tax if empty?(order.tax_address) || order.line_items.none?
         return no_tax unless taxable_address? order.tax_address
 
         cache do
@@ -150,6 +150,12 @@ def shipping_tax_label(shipment, shipping_tax)
       def line_item_tax_label(taxjar_line_item, spree_line_item)
         SuperGood::SolidusTaxJar.line_item_tax_label_maker.(taxjar_line_item, spree_line_item)
       end
+
+      def empty?(tax_address)
+        tax_address
+          .attributes
+          .except('id', 'created_at', 'updated_at', 'country_id').all? { |_, v| v.nil? }
+      end
     end
   end
 end