Trac importer: handle nil usernames.

git-svn-id: http://redmine.rubyforge.org/svn/trunk@1125 e93f8b46-1217-0410-a6f0-8f06a7374b81

lib/tasks/migrate_from_trac.rake

@@ -157,6 +157,8 @@ namespace :redmine do
       end
       
       def self.find_or_create_user(username, project_member = false)
+        return User.anonymous if username.blank?
+        
         u = User.find_by_login(username)
         if !u
           # Create a new user if not found