form_group with empty collection iteration produces NoMethodError

```
<%= bootstrap_form_tag(url: "/asd", layout: :inline, method: :get) do |f| %>
  <h1>test</h1>
  <%= f.form_group do %>
    <% [].each do |x| %>
      <%= x %>
    <% end %>
  <% end %>
<% end %>
```

Throws `undefined method 'concat' for nil:NilClass`
-  ruby 2.1.1p76 (2014-02-24 revision 45161) [x86_64-linux-gnu]
-  rails 4.1.1
-  bootstrap_form 2.1.1
