unmemoize read_ability

since it's only called once now in make_recipient

app/services/notification_recipient_service.rb

@@ -87,7 +87,6 @@ module NotificationRecipientService
       end
 
       def read_ability
-        @read_ability ||=
         case target
         when Issuable
           :"read_#{target.to_ability_name}"
@@ -308,7 +307,9 @@ module NotificationRecipientService
       end
 
       def read_ability
-        @read_ability ||= :"read_#{target.class.model_name.name.underscore}"
+        return nil if target.nil?
+
+        :"read_#{target.class.model_name.name.underscore}"
       end
 
       def build!