[PATCH] error_messages_on returning content for methods without errors

When using error_messages_on in a form, a <div class="formErrors"></div> element is being rendered into the form for methods without any errors on them.

Given the form

<% form_for @object do |f| %>
  <%= f.label :name %>
  <%= f.text_field :name %>
  <%= f.error_messages_on :name %>
<% end %>

I'm getting this HTML

<form action="/accounts" class="new_account" id="new_account" method="post"><div style="margin:0;padding:0;display:inline"><input name="authenticity_token" type="hidden" value="BOt7Iq1ZyBtEYyqbb0DTAj2tNJPYudQA3VmXesekztI=" /></div>
  <label for="account_name">Name</label>
  <input id="account_name" name="account[name]" size="30" type="text" />
  <div class="formError"></div>
</form>

I think this might be related to errors[:name] returning an empty array instead of nil as it used to

Loading development environment (Rails 3.0.0.beta1)
>> Account.new.errors[:name]
=> []

I've attached a patch which replicates this, I'm not sure how it should be fixed though. Either ActiveModel::Errors#[] needs to be patched to return nil for methods without errors or else the ActionView::Helpers::ActiveModelHelper#error_messages_on needs to be changed to check that the errors on the method are not empty.