Prevent "can't convert nil into String" error when :sort_order param is not present.

git-svn-id: http://redmine.rubyforge.org/svn/trunk@1410 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/helpers/sort_helper.rb

@@ -83,7 +83,7 @@ module SortHelper
   # Use this to sort the controller's table items collection.
   #
   def sort_clause()
-    session[@sort_name][:key] + ' ' + session[@sort_name][:order]
+    session[@sort_name][:key] + ' ' + (session[@sort_name][:order] || 'ASC')
   end
 
   # Returns a link which sorts by the named column.