Fix notification rendering on card closure events

where the closure no longer exists, possibly due to being
reopened.

Previously, fragment caching generally prevented any problems, but if
the cache is busted for any reason (like Andy did in c7eeb56c), then
the previous logic would raise an exception like "undefined method
'reason' for nil".

So, if the closure no longer exists, let's just note that it was
closed, and by who.

app/helpers/notifications_helper.rb

@@ -10,7 +10,12 @@ module NotificationsHelper
     name = event.creator.name
 
     case event_notification_action(event)
-    when "card_closed" then "Closed as “#{event.card.closure.reason}” by #{name}"
+    when "card_closed"
+      if event.card.closure
+        "Closed as “#{event.card.closure.reason}” by #{name}"
+      else
+        "Closed by #{name}"
+      end
     when "card_reopened" then "Reopened by #{name}"
     when "card_published" then "Added by #{name}"
     when "comment_created" then comment_notification_body(event)