Make Comment find_since_last_login scope robust against nil value

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

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