Feature/attributes table for nil

If you pass `nil` to `attributes_table_for` it raises exception. So app have a lot of boilerplate code:

```ruby
if user.profile.present?
  attributes_table_for user.profile
else
  span class: 'empty' do 
    'Empty'
  end
end
```

This PR implements such behavior for `attributes_table_for`. So it render Empty tag if `nil` given as attribute