Fixed: error on Trac import when :due attribute is nil (#1164).

git-svn-id: http://redmine.rubyforge.org/svn/trunk@1406 e93f8b46-1217-0410-a6f0-8f06a7374b81

lib/tasks/migrate_from_trac.rake

@@ -92,7 +92,7 @@ namespace :redmine do
         set_table_name :milestone
         
         def due
-          if read_attribute(:due) > 0
+          if read_attribute(:due) && read_attribute(:due) > 0
             Time.at(read_attribute(:due)).to_date
           else
             nil