Fixes a bug where error_messages_for was returning an empty div [#4048 state:resolved] (ht: Geoff Garside)

actionpack/lib/action_view/helpers/active_model_helper.rb

@@ -127,7 +127,7 @@ module ActionView
         object = convert_to_model(object)
 
         if (obj = (object.respond_to?(:errors) ? object : instance_variable_get("@#{object}"))) &&
-          (errors = obj.errors[method])
+          (errors = obj.errors[method]).presence
           content_tag("div",
             "#{options[:prepend_text]}#{ERB::Util.h(errors.first)}#{options[:append_text]}".html_safe,
             :class => options[:css_class]

actionpack/test/template/active_model_helper_test.rb

@@ -266,6 +266,10 @@ class ActiveModelHelperTest < ActionView::TestCase
     assert_dom_equal "<div class=\"differentError\">beforecan't be emptyafter</div>", error_message_on(:post, :author_name, :css_class => 'differentError', :prepend_text => 'before', :append_text => 'after')
   end
 
+  def test_error_message_on_handles_empty_errors
+    assert_equal "", error_message_on(@post, :tag)
+  end
+
   def test_error_messages_for_many_objects
     assert_dom_equal %(<div class="errorExplanation" id="errorExplanation"><h2>2 errors prohibited this post from being saved</h2><p>There were problems with the following fields:</p><ul><li>Author name can't be empty</li><li>User email can't be empty</li></ul></div>), error_messages_for("post", "user")