correct unread resetting to handle nulls

Note, to avoid race conditions we are setting last_unread to 10 minutes ago
if there is nothing unread.

This is safer in case of in progress transactions
we don't want to lose unread for any window of time.

app/models/user_stat.rb

@@ -11,9 +11,9 @@ class UserStat < ActiveRecord::Base
   end
 
   def self.update_first_unread(last_seen, limit: 10_000)
-    DB.exec(<<~SQL, min_date: last_seen, limit: limit)
+    DB.exec(<<~SQL, min_date: last_seen, limit: limit, now: 10.minutes.ago)
       UPDATE user_stats us
-      SET first_unread_at = Y.min_date
+      SET first_unread_at = COALESCE(Y.min_date, :now)
       FROM (
         SELECT u1.id user_id,
            X.min min_date