Use Order#email to show the order's email in new admin

`spree_orders` table has the column `email` which stores the email of
guest orders or users email for non guest orders. We should use
that in the new admin to display the email so that guest orders work
as well.

admin/app/components/solidus_admin/orders/index/component.rb

@@ -143,7 +143,7 @@ def customer_column
       col: { class: "w-[400px]" },
       header: :customer,
       data: ->(order) do
-        customer_email = order.user&.email
+        customer_email = order.email
         content_tag :div, String(customer_email)
       end
     }

admin/spec/features/orders/index_spec.rb

@@ -11,6 +11,7 @@
     visit "/admin/orders"
     click_on "In Progress"
 
+    expect(page).to have_content("[email protected]")
     expect(page).to have_content("R123456789")
     expect(page).to have_content("$19.99")
     expect(page).to be_axe_clean