Allow user to specify the partial which will be rendered if the collection is empty or nil

Hi,

I've think about this a while ago, and it seems like in the real app I've to keep doing this:

<%- unless @posts.empty? %>
  <%= render @posts %>
<%- else -%>
  <%= render "no_post" %>
<%- end -%>

I think there should be the way to accomplish this better. So, I propose the :if_empty option, which user can be specify the partial name to be rendered if the collection is empty. So the code portion about could become this:

<%= render :partial => @posts, :if_empty => "no_post" %>

I've also take it further and add support for :if_empty and :spacer_template in the shorthand version. Those options would be passed in the locals hash in the shorthand version, and would only be use if the first argument is the collection. So, you can do this

<%= render @posts, :if_empty => "no_post", :spacer_template => "post_spacer" %>

I think this will definitely make the collection render in the view looks nicer. I've separate the patch into two parts: the first one with the :if_empty addition only, and the second one for shorthand addition. In case you don't like the way I add to the shorthand, I would love to have at least the first patch got passed.

I'd love to see this got applied to both master and 3-0-stable.

Feedback and suggestion are always welcome. Thanks a lot.