Fix InvoicesHelper#display_entity

The invoice.entity might be nil, for example when a future membership has been deleted.

https://appsignal.com/csa-admin/sites/672e0b61d2a5e4b5197f10e4/exceptions/incidents/401/samples/timestamp/2025-01-28T17:05:27Z

app/helpers/invoices_helper.rb

@@ -10,7 +10,7 @@ def entity_type_collection
   def display_entity(invoice, link: true)
     if link && invoice.entity
       auto_link invoice.entity
-    elsif invoice.entity_type == "Membership"
+    elsif invoice.entity.is_a?(Membership)
       t("billing.membership_entity",
         fiscal_year: invoice.entity.fiscal_year,
         number: invoice.memberships_amount_description[/#\d+\z/])