Revert "Merge pull request #15312 from JuanitoFatas/action_view/asset_path"

This reverts commit 21ec7fef.

Per #16911, there's actually a good reason for a "blank" value to have a
useful `to_s` here. So let's also add some tests to prove that.

actionview/lib/action_view/helpers/asset_url_helper.rb

@@ -121,8 +121,8 @@ module AssetUrlHelper
       #   asset_path "application", type: :stylesheet     # => /assets/application.css
       #   asset_path "http://www.example.com/js/xmlhr.js" # => http://www.example.com/js/xmlhr.js
       def asset_path(source, options = {})
-        return "" unless source.present?
         source = source.to_s
+        return "" unless source.present?
         return source if source =~ URI_REGEXP
 
         tail, source = source[/([\?#].+)$/], source.sub(/([\?#].+)$/, '')

actionview/test/template/asset_tag_helper_test.rb

@@ -535,6 +535,17 @@ def test_should_not_modify_source_string
     assert_equal copy, source
   end
 
+  class PlaceholderImage
+    def blank?; true; end
+    def to_s; 'no-image-yet.png'; end
+  end
+  def test_image_tag_with_blank_placeholder
+    assert_equal '<img alt="" src="/images/no-image-yet.png" />', image_tag(PlaceholderImage.new, alt: "")
+  end
+  def test_image_path_with_blank_placeholder
+    assert_equal '/images/no-image-yet.png', image_path(PlaceholderImage.new)
+  end
+
   def test_image_path_with_asset_host_proc_returning_nil
     @controller.config.asset_host = Proc.new do |source|
       unless source.end_with?("tiff")