Fixed: LDAP authentication crashes if one of the LDAP attributes name is left blank on the LDAP setup screen.
When not checking "On-the-fly" register, the 3 optional fields can now be safely left blank.

git-svn-id: http://redmine.rubyforge.org/svn/trunk@854 e93f8b46-1217-0410-a6f0-8f06a7374b81