Fix nil error when viewing suspended domain in admin UI (#17765)

app/models/instance.rb

@@ -32,8 +32,12 @@ class Instance < ApplicationRecord
     @delivery_failure_tracker ||= DeliveryFailureTracker.new(domain)
   end
 
+  def purgeable?
+    unavailable? || domain_block&.suspend?
+  end
+
   def unavailable?
-    unavailable_domain.present? || domain_block&.suspend?
+    unavailable_domain.present?
   end
 
   def failing?

app/views/admin/instances/show.html.haml

@@ -89,7 +89,7 @@
         = t('admin.instances.availability.failures_recorded', count: @instance.delivery_failure_tracker.days)
         = link_to t('admin.instances.delivery.clear'), clear_delivery_errors_admin_instance_path(@instance), data: { confirm: t('admin.accounts.are_you_sure'), method: :post } unless @instance.exhausted_deliveries_days.empty?
 
-- if @instance.unavailable?
+- if @instance.purgeable?
   %p= t('admin.instances.purge_description_html')
 
   = link_to t('admin.instances.purge'), admin_instance_path(@instance), data: { confirm: t('admin.instances.confirm_purge'), method: :delete }, class: 'button button--destructive'