Make Comment find_since_last_login scope robust against nil value

to fix production ArgumentError: bad value for range. Fixes #807.

app/models/comment.rb

@@ -41,7 +41,11 @@ class Comment < ActiveRecord::Base
   }
 
   scope :find_since_last_login, lambda { |user|
-    where(created_at: (user.last_sign_in_at..Time.now)).order(created_at: :desc)
+    if user.last_sign_in_at
+      where(created_at: (user.last_sign_in_at..Time.now)).order(created_at: :desc)
+    else
+      none
+    end
   }
   # Helper class method to look up a commentable object
   # given the commentable class name and id

spec/models/comment_spec.rb

@@ -0,0 +1,10 @@
+require 'spec_helper'
+
+describe Commercial do
+
+  describe '.find_since_last_login' do
+    it 'returns none if last_sign_in_at is nil' do
+      expect(Comment.find_since_last_login(create(:user))).to eq(Comment.none)
+    end
+  end
+end