Make asset timestamping safe to use on assets that don't exist

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@4103 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

actionpack/lib/action_view/helpers/asset_tag_helper.rb

@@ -160,7 +160,7 @@ module ActionView
         
         def rails_asset_id(source)
           ENV["RAILS_ASSET_ID"] || 
-            File.stat(RAILS_ROOT + "/public/#{source}").mtime.to_i.to_s
+            File.mtime("#{RAILS_ROOT}/public/#{source}").to_i.to_s rescue ""
         end
     end
   end